error-handling: On-disk config file backend

Includes:

	- Proper error reporting when encountering syntax errors in a
	config file (file, line number, column).

	- Rewritten `config_write`, now with 99% less goto-spaghetti

	- Error state in `git_filebuf`: filebuf write functions no longer
	need to be checked for error returns. If any of the writes performed
	on a buffer fail, the last call to `git_filebuf_commit` or
	`git_filebuf_hash` will fail accordingly and set the appropiate error
	message. Baller!

10 files changed, 480 insertions, 486 deletions

diff --git a/include/git2/errors.h b/include/git2/errors.h
index bc420d1d4..085dd52f0 100644
--- a/include/git2/errors.h
+++ b/include/git2/errors.h
@@ -127,14 +127,9 @@ typedef enum {
 	GITERR_ZLIB,
 	GITERR_REPOSITORY,
 	GITERR_CONFIG,
+	GITERR_REGEX,
 } git_error_class;
 
-#define GITERR_CHECK_ALLOC(ptr) if (ptr == NULL) { return -1; }
-
-GIT_EXTERN(void) giterr_set_oom(void);
-GIT_EXTERN(void) giterr_set(int error_class, const char *string, ...);
-GIT_EXTERN(void) giterr_clear(void);
-
 /**
  * Return a detailed error string with the latest error
  * that occurred in the library.
diff --git a/src/common.h b/src/common.h
index 0b4dc2e49..30757de70 100644
--- a/src/common.h
+++ b/src/common.h
@@ -46,6 +46,8 @@
 #include "thread-utils.h"
 #include "bswap.h"
 
+#include <regex.h>
+
 extern void git___throw(const char *, ...) GIT_FORMAT_PRINTF(1, 2);
 #define git__throw(error, ...) \
 	(git___throw(__VA_ARGS__), error)
@@ -54,6 +56,16 @@ extern void git___rethrow(const char *, ...) GIT_FORMAT_PRINTF(1, 2);
 #define git__rethrow(error, ...) \
 	(git___rethrow(__VA_ARGS__), error)
 
+
+#define GITERR_CHECK_ALLOC(ptr) if (ptr == NULL) { return -1; }
+
+void giterr_set_oom(void);
+void giterr_set(int error_class, const char *string, ...);
+void giterr_clear(void);
+void giterr_set_str(int error_class, const char *string);
+void giterr_set_regex(const regex_t *regex, int error_code);
+
+
 #include "util.h"
 
 
diff --git a/src/config.c b/src/config.c
index 850c9b15f..77598d6a6 100644
--- a/src/config.c
+++ b/src/config.c
@@ -401,13 +401,15 @@ int git_config_get_string(git_config *cfg, const char *name, const char **out)
 		if (ret == 0)
 			return 0;
 
+		/* File backend doesn't set error message on variable
+		 * not found */
 		if (ret == GIT_ENOTFOUND)
 			continue;
 
 		return ret;
 	}
 
-	giterr_set(GITERR_CONFIG, "Config value '%s' not found", name);
+	giterr_set(GITERR_CONFIG, "Config variable '%s' not found", name);
 	return GIT_ENOTFOUND;
 }
 
diff --git a/src/config_file.c b/src/config_file.c
index 3c7c593ec..077e2c03f 100644
--- a/src/config_file.c
+++ b/src/config_file.c
@@ -86,6 +86,12 @@ static int config_parse(diskfile_backend *cfg_file);
 static int parse_variable(diskfile_backend *cfg, char **var_name, char **var_value);
 static int config_write(diskfile_backend *cfg, const char *key, const regex_t *preg, const char *value);
 
+static void set_parse_error(diskfile_backend *backend, int col, const char *error_str)
+{
+	giterr_set(GITERR_CONFIG, "Failed to parse config file: %s (in %s:%d, column %d)",
+		error_str, backend->file_path, backend->reader.line_number, col);
+}
+
 static void cvar_free(cvar_t *var)
 {
 	if (var == NULL)
@@ -104,15 +110,16 @@ static int normalize_name(const char *in, char **out)
 	assert(in && out);
 
 	name = git__strdup(in);
-	if (name == NULL)
-		return GIT_ENOMEM;
+	GITERR_CHECK_ALLOC(name);
 
 	fdot = strchr(name, '.');
 	ldot = strrchr(name, '.');
 
 	if (fdot == NULL || ldot == NULL) {
 		git__free(name);
-		return git__throw(GIT_EINVALIDARGS, "Bad format. No dot in '%s'", in);
+		giterr_set(GITERR_CONFIG,
+			"Invalid variable name: '%s'", in);
+		return -1;
 	}
 
 	/* Downcase up to the first dot and after the last one */
@@ -120,7 +127,7 @@ static int normalize_name(const char *in, char **out)
 	git__strtolower(ldot);
 
 	*out = name;
-	return GIT_SUCCESS;
+	return 0;
 }
 
 static void free_vars(git_hashtable *values)
@@ -143,37 +150,28 @@ static void free_vars(git_hashtable *values)
 
 static int config_open(git_config_file *cfg)
 {
-	int error;
+	int res;
 	diskfile_backend *b = (diskfile_backend *)cfg;
 
 	b->values = git_hashtable_alloc (20, git_hash__strhash_cb, git_hash__strcmp_cb);
-	if (b->values == NULL)
-		return GIT_ENOMEM;
+	GITERR_CHECK_ALLOC(b->values);
 
 	git_buf_init(&b->reader.buffer, 0);
-	error = git_futils_readbuffer(&b->reader.buffer, b->file_path);
+	res = git_futils_readbuffer(&b->reader.buffer, b->file_path);
 
 	/* It's fine if the file doesn't exist */
-	if (error == GIT_ENOTFOUND)
-		return GIT_SUCCESS;
-
-	if (error < GIT_SUCCESS)
-		goto cleanup;
-
-	error = config_parse(b);
-	if (error < GIT_SUCCESS)
-		goto cleanup;
-
-	git_buf_free(&b->reader.buffer);
-
-	return GIT_SUCCESS;
+	if (res == GIT_ENOTFOUND)
+		return 0;
+
+	if (res < 0 || config_parse(b) <  0) {
+		free_vars(b->values);
+		b->values = NULL;
+		git_buf_free(&b->reader.buffer);
+		return -1;
+	}
 
- cleanup:
-	free_vars(b->values);
-	b->values = NULL;
 	git_buf_free(&b->reader.buffer);
-
-	return git__rethrow(error, "Failed to open config");
+	return 0;
 }
 
 static void backend_free(git_config_file *_backend)
@@ -184,42 +182,37 @@ static void backend_free(git_config_file *_backend)
 		return;
 
 	git__free(backend->file_path);
-
 	free_vars(backend->values);
-
 	git__free(backend);
 }
 
 static int file_foreach(git_config_file *backend, int (*fn)(const char *, const char *, void *), void *data)
 {
-	int ret = GIT_SUCCESS;
-	cvar_t *var;
 	diskfile_backend *b = (diskfile_backend *)backend;
+	cvar_t *var;
 	const char *key;
 
 	GIT_HASHTABLE_FOREACH(b->values, key, var,
-	      do {
-		      ret = fn(key, var->value, data);
-		      if (ret)
-			      break;
-		      var = CVAR_LIST_NEXT(var);
-	      } while (var != NULL);
-	)
+		do {
+			if (fn(key, var->value, data) < 0)
+				break;
 
+			var = CVAR_LIST_NEXT(var);
+		} while (var != NULL);
+	)
 
-	return ret;
+	return 0;
 }
 
 static int config_set(git_config_file *cfg, const char *name, const char *value)
 {
 	cvar_t *var = NULL;
 	cvar_t *existing = NULL, *old_value = NULL;
-	int error = GIT_SUCCESS;
 	diskfile_backend *b = (diskfile_backend *)cfg;
 	char *key;
 
-	if ((error = normalize_name(name, &key)) < GIT_SUCCESS)
-		return git__rethrow(error, "Failed to normalize variable name '%s'", name);
+	if (normalize_name(name, &key) < 0)
+		return -1;
 
 	/*
 	 * Try to find it in the existing values and update it if it
@@ -227,15 +220,18 @@ static int config_set(git_config_file *cfg, const char *name, const char *value)
 	 */
 	existing = git_hashtable_lookup(b->values, key);
 	if (existing != NULL) {
-		char *tmp;
+		char *tmp = NULL;
 
 		git__free(key);
-		if (existing->next != NULL)
-			return git__throw(GIT_EINVALIDARGS, "Multivar incompatible with simple set");
+		if (existing->next != NULL) {
+			giterr_set(GITERR_CONFIG, "Multivar incompatible with simple set");
+			return -1;
+		}
 
-		tmp = value ? git__strdup(value) : NULL;
-		if (tmp == NULL && value != NULL)
-			return GIT_ENOMEM;
+		if (value) {
+			tmp = git__strdup(value);
+			GITERR_CHECK_ALLOC(tmp);
+		}
 
 		git__free(existing->value);
 		existing->value = tmp;
@@ -244,32 +240,29 @@ static int config_set(git_config_file *cfg, const char *name, const char *value)
 	}
 
 	var = git__malloc(sizeof(cvar_t));
-	if (var == NULL)
-		return GIT_ENOMEM;
+	GITERR_CHECK_ALLOC(var);
 
 	memset(var, 0x0, sizeof(cvar_t));
 
 	var->key = key;
+	var->value = NULL;
 
-	var->value = value ? git__strdup(value) : NULL;
-	if (var->value == NULL && value != NULL) {
-		error = GIT_ENOMEM;
-		goto out;
+	if (value) {
+		var->value = git__strdup(value);
+		GITERR_CHECK_ALLOC(var->value);
 	}
 
-	error = git_hashtable_insert2(b->values, key, var, (void **)&old_value);
-	if (error < GIT_SUCCESS)
-		goto out;
+	if (git_hashtable_insert2(b->values, key, var, (void **)&old_value) < 0)
+		return -1;
 
 	cvar_free(old_value);
 
-	error = config_write(b, key, NULL, value);
-
- out:
-	if (error < GIT_SUCCESS)
+	if (config_write(b, key, NULL, value) < 0) {
 		cvar_free(var);
+		return -1;
+	}
 
-	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to set config value");
+	return 0;
 }
 
 /*
@@ -278,171 +271,172 @@ static int config_set(git_config_file *cfg, const char *name, const char *value)
 static int config_get(git_config_file *cfg, const char *name, const char **out)
 {
 	cvar_t *var;
-	int error = GIT_SUCCESS;
 	diskfile_backend *b = (diskfile_backend *)cfg;
 	char *key;
 
-	if ((error = normalize_name(name, &key)) < GIT_SUCCESS)
-		return error;
+	if (normalize_name(name, &key) < 0)
+		return -1;
 
 	var = git_hashtable_lookup(b->values, key);
 	git__free(key);
 
+	/* no error message; the config system will write one */
 	if (var == NULL)
-		return git__throw(GIT_ENOTFOUND, "Variable '%s' not found", name);
+		return GIT_ENOTFOUND;
 
 	*out = var->value;
-
-	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to get config value for %s", name);
+	return 0;
 }
 
-static int config_get_multivar(git_config_file *cfg, const char *name, const char *regexp, int (*fn)(const char *, void *), void *data)
+static int config_get_multivar(
+	git_config_file *cfg,
+	const char *name,
+	const char *regex_str,
+	int (*fn)(const char *, void *),
+	void *data)
 {
 	cvar_t *var;
-	int error = GIT_SUCCESS;
 	diskfile_backend *b = (diskfile_backend *)cfg;
 	char *key;
-	regex_t preg;
 
-	if ((error = normalize_name(name, &key)) < GIT_SUCCESS)
-		return error;
+	if (normalize_name(name, &key) < 0)
+		return -1;
 
 	var = git_hashtable_lookup(b->values, key);
 	git__free(key);
 
 	if (var == NULL)
-		return git__throw(GIT_ENOTFOUND, "Variable '%s' not found", name);
+		return GIT_ENOTFOUND;
 
-	if (regexp != NULL) {
-		error = regcomp(&preg, regexp, REG_EXTENDED);
-		if (error < 0)
-			return git__throw(GIT_EINVALIDARGS, "Failed to compile regex");
-	}
+	if (regex_str != NULL) {
+		regex_t regex;
+		int result;
 
-	do {
-		if (regexp == NULL || !regexec(&preg, var->value, 0, NULL, 0)) {
-			error = fn(var->value, data);
-			if (error < GIT_SUCCESS)
-				goto exit;
+		/* regex matching; build the regex */
+		result = regcomp(&regex, regex_str, REG_EXTENDED);
+		if (result < 0) {
+			giterr_set_regex(&regex, result);
+			return -1;
 		}
 
-		var = var->next;
-	} while (var != NULL);
+		/* and throw the callback only on the variables that
+		 * match the regex */
+		do {
+			if (regexec(&regex, var->value, 0, NULL, 0) == 0) {
+				/* early termination by the user is not an error;
+				 * just break and return successfully */
+				if (fn(var->value, data) < 0)
+					break;
+			}
 
- exit:
-	if (regexp != NULL)
-		regfree(&preg);
-	return error;
+			var = var->next;
+		} while (var != NULL);
+	} else {
+		/* no regex; go through all the variables */
+		do {
+			/* early termination by the user is not an error;
+			 * just break and return successfully */
+			if (fn(var->value, data) < 0)
+				break;
+
+			var = var->next;
+		} while (var != NULL);
+	}
+
+	return 0;
 }
 
 static int config_set_multivar(git_config_file *cfg, const char *name, const char *regexp, const char *value)
 {
-	int error, replaced = 0;
+	int replaced = 0;
 	cvar_t *var, *newvar;
 	diskfile_backend *b = (diskfile_backend *)cfg;
 	char *key;
 	regex_t preg;
+	int result;
 
-	if (regexp == NULL)
-		return git__throw(GIT_EINVALIDARGS, "No regex supplied");
+	assert(regexp);
 
-	if ((error = normalize_name(name, &key)) < GIT_SUCCESS)
-		return error;
+	if (normalize_name(name, &key) < 0)
+		return -1;
 
 	var = git_hashtable_lookup(b->values, key);
-	if (var == NULL) {
-		free(key);
-		return git__throw(GIT_ENOTFOUND, "Variable '%s' not found", name);
-	}
+	if (var == NULL)
+		return GIT_ENOTFOUND;
 
-	error = regcomp(&preg, regexp, REG_EXTENDED);
-	if (error < 0) {
+	result = regcomp(&preg, regexp, REG_EXTENDED);
+	if (result < 0) {
 		free(key);
-		return git__throw(GIT_EINVALIDARGS, "Failed to compile regex");
+		giterr_set_regex(&preg, result);
+		return -1;
 	}
 
-	do {
-		if (!regexec(&preg, var->value, 0, NULL, 0)) {
+	for (;;) {
+		if (regexec(&preg, var->value, 0, NULL, 0) == 0) {
 			char *tmp = git__strdup(value);
-			if (tmp == NULL) {
-				error = GIT_ENOMEM;
-				goto exit;
-			}
+			GITERR_CHECK_ALLOC(tmp);
 
 			free(var->value);
 			var->value = tmp;
 			replaced = 1;
 		}
 
-		if (var->next != NULL)
-			var = var->next;
-		else
+		if (var->next == NULL)
 			break;
-	} while (var != NULL);
+
+		var = var->next;
+	}
 
 	/* If we've reached the end of the variables and we haven't found it yet, we need to append it */
 	if (!replaced) {
 		newvar = git__malloc(sizeof(cvar_t));
-		if (newvar == NULL) {
-			error = GIT_ENOMEM;
-			goto exit;
-		}
+		GITERR_CHECK_ALLOC(newvar);
 
 		memset(newvar, 0x0, sizeof(cvar_t));
+
 		newvar->key = git__strdup(var->key);
-		if (newvar->key == NULL) {
-			error = GIT_ENOMEM;
-			goto exit;
-		}
+		GITERR_CHECK_ALLOC(newvar->key);
+
 		newvar->value = git__strdup(value);
-		if (newvar->value == NULL) {
-			error = GIT_ENOMEM;
-			goto exit;
-		}
+		GITERR_CHECK_ALLOC(newvar->value);
 
 		var->next = newvar;
 	}
 
-	error = config_write(b, key, &preg, value);
-	if (error < GIT_SUCCESS) {
-		error = git__rethrow(error, "Failed to update value in file");
-		goto exit;
-	}
+	result = config_write(b, key, &preg, value);
 
- exit:
 	free(key);
 	regfree(&preg);
-	return error;
+
+	return result;
 }
 
 static int config_delete(git_config_file *cfg, const char *name)
 {
-	int error;
-	const cvar_t *var;
-	cvar_t *old_value;
+	cvar_t *var;
 	diskfile_backend *b = (diskfile_backend *)cfg;
 	char *key;
+	int result;
 
-	if ((error = normalize_name(name, &key)) < GIT_SUCCESS)
-		return error;
+	if (normalize_name(name, &key) < 0)
+		return -1;
 
 	var = git_hashtable_lookup(b->values, key);
 	free(key);
 
 	if (var == NULL)
-		return git__throw(GIT_ENOTFOUND, "Variable '%s' not found", name);
-
-	if (var->next != NULL)
-		return git__throw(GIT_EINVALIDARGS, "Multivar incompatible with simple delete");
+		return GIT_ENOTFOUND;
 
+	if (var->next != NULL) {
+		giterr_set(GITERR_CONFIG, "Cannot delete multivar with a single delete");
+		return -1;
+	}
 
-	if ((error = git_hashtable_remove2(b->values, var->key, (void **)&old_value)) < GIT_SUCCESS)
-		return git__rethrow(error, "Failed to remove %s from hashtable", key);
-
-	error = config_write(b, var->key, NULL, NULL);
-	cvar_free(old_value);
+	git_hashtable_remove(b->values, var->key);
+	result = config_write(b, var->key, NULL, NULL);
 
-	return error;
+	cvar_free(var);
+	return result;
 }
 
 int git_config_file__ondisk(git_config_file **out, const char *path)
@@ -450,16 +444,12 @@ int git_config_file__ondisk(git_config_file **out, const char *path)
 	diskfile_backend *backend;
 
 	backend = git__malloc(sizeof(diskfile_backend));
-	if (backend == NULL)
-		return GIT_ENOMEM;
+	GITERR_CHECK_ALLOC(backend);
 
 	memset(backend, 0x0, sizeof(diskfile_backend));
 
 	backend->file_path = git__strdup(path);
-	if (backend->file_path == NULL) {
-		git__free(backend);
-		return GIT_ENOMEM;
-	}
+	GITERR_CHECK_ALLOC(backend->file_path);
 
 	backend->parent.open = config_open;
 	backend->parent.get = config_get;
@@ -472,7 +462,7 @@ int git_config_file__ondisk(git_config_file **out, const char *path)
 
 	*out = (git_config_file *)backend;
 
-	return GIT_SUCCESS;
+	return 0;
 }
 
 static int cfg_getchar_raw(diskfile_backend *cfg)
@@ -621,12 +611,11 @@ GIT_INLINE(int) config_keychar(int c)
 	return isalnum(c) || c == '-';
 }
 
-static int parse_section_header_ext(const char *line, const char *base_name, char **section_name)
+static int parse_section_header_ext(diskfile_backend *cfg, const char *line, const char *base_name, char **section_name)
 {
 	int c, rpos;
 	char *first_quote, *last_quote;
 	git_buf buf = GIT_BUF_INIT;
-	int error = GIT_SUCCESS;
 	int quote_marks;
 	/*
 	 * base_name is what came before the space. We should be at the
@@ -637,8 +626,10 @@ static int parse_section_header_ext(const char *line, const char *base_name, cha
 	first_quote = strchr(line, '"');
 	last_quote = strrchr(line, '"');
 
-	if (last_quote - first_quote == 0)
-		return git__throw(GIT_EOBJCORRUPTED, "Failed to parse ext header. There is no final quotation mark");
+	if (last_quote - first_quote == 0) {
+		set_parse_error(cfg, 0, "Missing closing quotation mark in section header");
+		return -1;
+	}
 
 	git_buf_grow(&buf, strlen(base_name) + last_quote - first_quote + 2);
 	git_buf_printf(&buf, "%s.", base_name);
@@ -655,27 +646,30 @@ static int parse_section_header_ext(const char *line, const char *base_name, cha
 	 */
 	do {
 		if (quote_marks == 2) {
-			puts("too many marks");
-			error = git__throw(GIT_EOBJCORRUPTED, "Falied to parse ext header. Text after closing quote");
-			goto out;
-
+			set_parse_error(cfg, rpos, "Unexpected text after closing quotes");
+			git_buf_free(&buf);
+			return -1;
 		}
 
 		switch (c) {
 		case '"':
 			++quote_marks;
 			continue;
+
 		case '\\':
 			c = line[rpos++];
+
 			switch (c) {
 			case '"':
 			case '\\':
 				break;
+
 			default:
-				error = git__throw(GIT_EOBJCORRUPTED, "Failed to parse ext header. Unsupported escape char \\%c", c);
-				goto out;
+				set_parse_error(cfg, rpos, "Unsupported escape sequence");
+				git_buf_free(&buf);
+				return -1;
 			}
-			break;
+
 		default:
 			break;
 		}
@@ -683,61 +677,53 @@ static int parse_section_header_ext(const char *line, const char *base_name, cha
 		git_buf_putc(&buf, c);
 	} while ((c = line[rpos++]) != ']');
 
-	*section_name = git__strdup(git_buf_cstr(&buf));
- out:
-	git_buf_free(&buf);
-
-	return error;
+	*section_name = git_buf_detach(&buf);
+	return 0;
 }
 
 static int parse_section_header(diskfile_backend *cfg, char **section_out)
 {
 	char *name, *name_end;
 	int name_length, c, pos;
-	int error = GIT_SUCCESS;
+	int result;
 	char *line;
 
 	line = cfg_readline(cfg);
 	if (line == NULL)
-		return GIT_ENOMEM;
+		return -1;
 
 	/* find the end of the variable's name */
 	name_end = strchr(line, ']');
 	if (name_end == NULL) {
 		git__free(line);
-		return git__throw(GIT_EOBJCORRUPTED, "Failed to parse header. Can't find header name end");
+		set_parse_error(cfg, 0, "Missing ']' in section header");
+		return -1;
 	}
 
 	name = (char *)git__malloc((size_t)(name_end - line) + 1);
-	if (name == NULL) {
-		git__free(line);
-		return GIT_ENOMEM;
-	}
+	GITERR_CHECK_ALLOC(name);
 
 	name_length = 0;
 	pos = 0;
 
 	/* Make sure we were given a section header */
 	c = line[pos++];
-	if (c != '[') {
-		error = git__throw(GIT_ERROR, "Failed to parse header. Didn't get section header. This is a bug");
-		goto error;
-	}
+	assert(c == '[');
 
 	c = line[pos++];
 
 	do {
 		if (isspace(c)){
 			name[name_length] = '\0';
-			error = parse_section_header_ext(line, name, section_out);
+			result = parse_section_header_ext(cfg, line, name, section_out);
 			git__free(line);
 			git__free(name);
-			return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to parse header");
+			return result;
 		}
 
 		if (!config_keychar(c) && c != '.') {
-			error = git__throw(GIT_EOBJCORRUPTED, "Failed to parse header. Wrong format on header");
-			goto error;
+			set_parse_error(cfg, pos, "Unexpected character in header");
+			goto fail_parse;
 		}
 
 		name[name_length++] = (char) tolower(c);
@@ -745,20 +731,21 @@ static int parse_section_header(diskfile_backend *cfg, char **section_out)
 	} while ((c = line[pos++]) != ']');
 
 	if (line[pos - 1] != ']') {
-		error = git__throw(GIT_EOBJCORRUPTED, "Failed to parse header. Config file ended unexpectedly");
-		goto error;
+		set_parse_error(cfg, pos, "Unexpected end of file");
+		goto fail_parse;
 	}
 
-	name[name_length] = 0;
 	git__free(line);
-	git__strtolower(name);
+
+	name[name_length] = 0;
 	*section_out = name;
-	return GIT_SUCCESS;
 
-error:
+	return 0;
+
+fail_parse:
 	git__free(line);
 	git__free(name);
-	return error;
+	return -1;
 }
 
 static int skip_bom(diskfile_backend *cfg)
@@ -766,7 +753,7 @@ static int skip_bom(diskfile_backend *cfg)
 	static const char utf8_bom[] = "\xef\xbb\xbf";
 
 	if (cfg->reader.buffer.size < sizeof(utf8_bom))
-		return GIT_SUCCESS;
+		return 0;
 
 	if (memcmp(cfg->reader.read_ptr, utf8_bom, sizeof(utf8_bom)) == 0)
 		cfg->reader.read_ptr += sizeof(utf8_bom);
@@ -775,7 +762,7 @@ static int skip_bom(diskfile_backend *cfg)
 		shit with the BoM
 	*/
 
-	return GIT_SUCCESS;
+	return 0;
 }
 
 /*
@@ -839,12 +826,13 @@ static void strip_comments(char *line)
 
 static int config_parse(diskfile_backend *cfg_file)
 {
-	int error = GIT_SUCCESS, c;
+	int c;
 	char *current_section = NULL;
 	char *var_name;
 	char *var_value;
 	cvar_t *var, *existing;
 	git_buf buf = GIT_BUF_INIT;
+	int result = 0;
 
 	/* Initialize the reading position */
 	cfg_file->reader.read_ptr = cfg_file->reader.buffer.ptr;
@@ -852,11 +840,11 @@ static int config_parse(diskfile_backend *cfg_file)
 
 	/* If the file is empty, there's nothing for us to do */
 	if (*cfg_file->reader.read_ptr == '\0')
-		return GIT_SUCCESS;
+		return 0;
 
 	skip_bom(cfg_file);
 
-	while (error == GIT_SUCCESS && !cfg_file->reader.eof) {
+	while (result == 0 && !cfg_file->reader.eof) {
 
 		c = cfg_peek(cfg_file, SKIP_WHITESPACE);
 
@@ -868,7 +856,7 @@ static int config_parse(diskfile_backend *cfg_file)
 		case '[': /* section header, new section begins */
 			git__free(current_section);
 			current_section = NULL;
-			error = parse_section_header(cfg_file, &current_section);
+			result = parse_section_header(cfg_file, &current_section);
 			break;
 
 		case ';':
@@ -877,16 +865,12 @@ static int config_parse(diskfile_backend *cfg_file)
 			break;
 
 		default: /* assume variable declaration */
-			error = parse_variable(cfg_file, &var_name, &var_value);
-
-			if (error < GIT_SUCCESS)
+			result = parse_variable(cfg_file, &var_name, &var_value);
+			if (result < 0)
 				break;
 
 			var = git__malloc(sizeof(cvar_t));
-			if (var == NULL) {
-				error = GIT_ENOMEM;
-				break;
-			}
+			GITERR_CHECK_ALLOC(var);
 
 			memset(var, 0x0, sizeof(cvar_t));
 
@@ -894,10 +878,8 @@ static int config_parse(diskfile_backend *cfg_file)
 			git_buf_printf(&buf, "%s.%s", current_section, var_name);
 			git__free(var_name);
 
-			if (git_buf_oom(&buf)) {
-				error = GIT_ENOMEM;
-				break;
-			}
+			if (git_buf_oom(&buf))
+				return -1;
 
 			var->key = git_buf_detach(&buf);
 			var->value = var_value;
@@ -905,7 +887,7 @@ static int config_parse(diskfile_backend *cfg_file)
 			/* Add or append the new config option */
 			existing = git_hashtable_lookup(cfg_file->values, var->key);
 			if (existing == NULL) {
-				error = git_hashtable_insert(cfg_file->values, var->key, var);
+				result = git_hashtable_insert(cfg_file->values, var->key, var);
 			} else {
 				while (existing->next != NULL) {
 					existing = existing->next;
@@ -918,13 +900,12 @@ static int config_parse(diskfile_backend *cfg_file)
 	}
 
 	git__free(current_section);
-
-	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to parse config");
+	return result;
 }
 
 static int write_section(git_filebuf *file, const char *key)
 {
-	int error;
+	int result;
 	const char *fdot, *ldot;
 	git_buf buf = GIT_BUF_INIT;
 
@@ -943,13 +924,14 @@ static int write_section(git_filebuf *file, const char *key)
 		git_buf_putc(&buf, '"');
 	}
 	git_buf_puts(&buf, "]\n");
+
 	if (git_buf_oom(&buf))
-		return GIT_ENOMEM;
+		return -1;
 
-	error = git_filebuf_write(file, git_buf_cstr(&buf), buf.size);
+	result = git_filebuf_write(file, git_buf_cstr(&buf), buf.size);
 	git_buf_free(&buf);
 
-	return error;
+	return result;
 }
 
 /*
@@ -957,50 +939,45 @@ static int write_section(git_filebuf *file, const char *key)
  */
 static int config_write(diskfile_backend *cfg, const char *key, const regex_t *preg, const char* value)
 {
-	int error = GIT_SUCCESS, c;
-	int section_matches = 0, last_section_matched = 0, preg_replaced = 0;
+	int result, c;
+	int section_matches = 0, last_section_matched = 0, preg_replaced = 0, write_trailer = 0;
+	const char *pre_end = NULL, *post_start = NULL, *data_start;
 	char *current_section = NULL, *section, *name, *ldot;
-	char *var_name, *var_value;
 	git_filebuf file = GIT_FILEBUF_INIT;
-	const char *pre_end = NULL, *post_start = NULL, *data_start;
 
 	/* We need to read in our own config file */
-	error = git_futils_readbuffer(&cfg->reader.buffer, cfg->file_path);
-	if (error < GIT_SUCCESS && error != GIT_ENOTFOUND) {
-		return git__rethrow(error, "Failed to read existing config file %s", cfg->file_path);
-	}
+	result = git_futils_readbuffer(&cfg->reader.buffer, cfg->file_path);
 
 	/* Initialise the reading position */
-	if (error == GIT_ENOTFOUND) {
-		error = GIT_SUCCESS;
+	if (result == GIT_ENOTFOUND) {
 		cfg->reader.read_ptr = NULL;
 		cfg->reader.eof = 1;
 		data_start = NULL;
 		git_buf_clear(&cfg->reader.buffer);
-	} else {
+	} else if (result == 0) {
 		cfg->reader.read_ptr = cfg->reader.buffer.ptr;
 		cfg->reader.eof = 0;
 		data_start = cfg->reader.read_ptr;
+	} else {
+		return -1; /* OS error when reading the file */
 	}
 
 	/* Lock the file */
-	error = git_filebuf_open(&file, cfg->file_path, 0);
-	if (error < GIT_SUCCESS)
-		return git__rethrow(error, "Failed to lock config file");
+	if (git_filebuf_open(&file, cfg->file_path, 0) < 0)
+		return -1;
 
 	skip_bom(cfg);
 	ldot = strrchr(key, '.');
 	name = ldot + 1;
 	section = git__strndup(key, ldot - key);
 
-	while (error == GIT_SUCCESS && !cfg->reader.eof) {
+	while (!cfg->reader.eof) {
 		c = cfg_peek(cfg, SKIP_WHITESPACE);
 
-		switch (c) {
-		case '\0': /* We've arrived at the end of the file */
+		if (c == '\0') { /* We've arrived at the end of the file */
 			break;
 
-		case '[': /* section header, new section begins */
+		} else if (c == '[') { /* section header, new section begins */
 			/*
 			 * We set both positions to the current one in case we
 			 * need to add a variable to the end of a section. In that
@@ -1009,23 +986,21 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 			 * default case will take care of updating them.
 			 */
 			pre_end = post_start = cfg->reader.read_ptr;
-			if (current_section)
-				git__free(current_section);
-			error = parse_section_header(cfg, &current_section);
-			if (error < GIT_SUCCESS)
-				break;
+
+			git__free(current_section);
+			if (parse_section_header(cfg, &current_section) < 0)
+				goto rewrite_fail;
 
 			/* Keep track of when it stops matching */
 			last_section_matched = section_matches;
 			section_matches = !strcmp(current_section, section);
-			break;
+		}
 
-		case ';':
-		case '#':
+		else if (c == ';' || c == '#') {
 			cfg_consume_line(cfg);
-			break;
+		}
 
-		default:
+		else {
 			/*
 			 * If the section doesn't match, but the last section did,
 			 * it means we need to add a variable (so skip the line
@@ -1039,67 +1014,54 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 			if (!section_matches) {
 				if (!last_section_matched) {
 					cfg_consume_line(cfg);
-					break;
+					continue;
 				}
 			} else {
-				int cmp = -1;
+				int has_matched = 0;
+				char *var_name, *var_value;
 
 				pre_end = cfg->reader.read_ptr;
-				if ((error = parse_variable(cfg, &var_name, &var_value)) == GIT_SUCCESS)
-					cmp = strcasecmp(name, var_name);
+				if (parse_variable(cfg, &var_name, &var_value) < 0)
+					goto rewrite_fail;
 
-				if (cmp == 0 && preg != NULL)
-					cmp = regexec(preg, var_value, 0, NULL, 0);
+				/* First try to match the name of the variable */
+				if (strcasecmp(name, var_name) == 0)
+					has_matched = 1;
+
+				/* If the name matches, and we have a regex to match the
+				 * value, try to match it */
+				if (has_matched && preg != NULL)
+					has_matched = (regexec(preg, var_value, 0, NULL, 0) == 0);
 
 				git__free(var_name);
 				git__free(var_value);
 
-				if (cmp != 0)
-					break;
+				/* if there is no match, keep going */
+				if (!has_matched)
+					continue;
 
 				post_start = cfg->reader.read_ptr;
 			}
 
-			/*
-			 * We've found the variable we wanted to change, so
-			 * write anything up to it
-			 */
+			/* We've found the variable we wanted to change, so
+			 * write anything up to it */
+			git_filebuf_write(&file, data_start, pre_end - data_start);
 			preg_replaced = 1;
-			error = git_filebuf_write(&file, data_start, pre_end - data_start);
-			if (error < GIT_SUCCESS) {
-				git__rethrow(error, "Failed to write the first part of the file");
-				break;
-			}
 
-			/*
-			 * Then replace the variable. If the value is NULL, it
-			 * means we want to delete it, so pretend everything went
-			 * fine
-			 */
-			if (value == NULL)
-				error = GIT_SUCCESS;
-			else
-				error = git_filebuf_printf(&file, "\t%s = %s\n", name, value);
-			if (error < GIT_SUCCESS) {
-				git__rethrow(error, "Failed to overwrite the variable");
-				break;
+			/* Then replace the variable. If the value is NULL, it
+			 * means we want to delete it, so don't write anything. */
+			if (value != NULL) {
+				git_filebuf_printf(&file, "\t%s = %s\n", name, value);
 			}
 
+			/* multiline variable? we need to keep reading lines to match */
 			if (preg != NULL) {
 				data_start = post_start;
 				continue;
 			}
 
-			/* And then the write out rest of the file */
-			error = git_filebuf_write(&file, post_start,
-						cfg->reader.buffer.size - (post_start - data_start));
-
-			if (error < GIT_SUCCESS) {
-				git__rethrow(error, "Failed to write the rest of the file");
-					break;
-			}
-
-			goto cleanup;
+			write_trailer = 1;
+			break; /* break from the loop */
 		}
 	}
 
@@ -1119,132 +1081,108 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 	 * want to write the rest of the file. Otherwise we need to write
 	 * out the whole file and then the new variable.
 	 */
-	if (preg_replaced) {
-		error = git_filebuf_printf(&file, "\n%s", data_start);
-		if (error < GIT_SUCCESS)
-			error = git__rethrow(error, "Failed to write the rest of the file");
-
-		goto cleanup;
-	}
+	if (write_trailer) {
+		/* Write out rest of the file */
+		git_filebuf_write(&file, post_start, cfg->reader.buffer.size - (post_start - data_start));
+	} else {
+		if (preg_replaced) {
+			git_filebuf_printf(&file, "\n%s", data_start);
+		} else {
+			git_filebuf_write(&file, cfg->reader.buffer.ptr, cfg->reader.buffer.size);
+
+			/* And now if we just need to add a variable */
+			if (!section_matches && write_section(&file, section) < 0)
+				goto rewrite_fail;
+
+			/* Sanity check: if we are here, and value is NULL, that means that somebody
+			 * touched the config file after our intial read. We should probably assert()
+			 * this, but instead we'll handle it gracefully with an error. */
+			if (value == NULL) {
+				giterr_set(GITERR_CONFIG,
+					"Race condition when writing a config file (a cvar has been removed)");
+				goto rewrite_fail;
+			}
 
-	error = git_filebuf_write(&file, cfg->reader.buffer.ptr, cfg->reader.buffer.size);
-	if (error < GIT_SUCCESS) {
-		git__rethrow(error, "Failed to write original config content");
-		goto cleanup;
+			git_filebuf_printf(&file, "\t%s = %s\n", name, value);
+		}
 	}
 
-	/* And now if we just need to add a variable */
-	if (section_matches) {
-		error = git_filebuf_printf(&file, "\t%s = %s\n", name, value);
-		goto cleanup;
-	}
+	git__free(section);
+	git__free(current_section);
 
-	/* Or maybe we need to write out a whole section */
-	error = write_section(&file, section);
-	if (error < GIT_SUCCESS)
-		git__rethrow(error, "Failed to write new section");
+	result = git_filebuf_commit(&file, GIT_CONFIG_FILE_MODE);
+	git_buf_free(&cfg->reader.buffer);
+	return result;
 
-	error = git_filebuf_printf(&file, "\t%s = %s\n", name, value);
- cleanup:
+rewrite_fail:
 	git__free(section);
 	git__free(current_section);
 
-	if (error < GIT_SUCCESS)
-		git_filebuf_cleanup(&file);
-	else
-		error = git_filebuf_commit(&file, GIT_CONFIG_FILE_MODE);
-
+	git_filebuf_cleanup(&file);
 	git_buf_free(&cfg->reader.buffer);
-	return error;
+	return -1;
 }
 
 static int is_multiline_var(const char *str)
 {
-	char *end = strrchr(str, '\0') - 1;
-
-	while (isspace(*end))
-		--end;
-
-	return *end == '\\';
+	const char *end = str + strlen(str);
+	return (end > str) && (end[-1] == '\\');
 }
 
-static int parse_multiline_variable(diskfile_backend *cfg, const char *first, char **out)
+static int parse_multiline_variable(diskfile_backend *cfg, git_buf *value)
 {
-	char *line = NULL, *end;
-	int error = GIT_SUCCESS, ret;
-	size_t len;
-	char *buf;
+	char *line = NULL;
 
 	/* Check that the next line exists */
 	line = cfg_readline(cfg);
 	if (line == NULL)
-		return GIT_ENOMEM;
+		return -1;
 
 	/* We've reached the end of the file, there is input missing */
 	if (line[0] == '\0') {
-		error = git__throw(GIT_EOBJCORRUPTED, "Failed to parse multiline var. File ended unexpectedly");
-		goto out;
+		set_parse_error(cfg, 0, "Unexpected end of file while parsing multine var");
+		git__free(line);
+		return -1;
 	}
 
 	strip_comments(line);
 
 	/* If it was just a comment, pretend it didn't exist */
 	if (line[0] == '\0') {
-		error = parse_multiline_variable(cfg, first, out);
-		goto out;
+		git__free(line);
+		return parse_multiline_variable(cfg, value);
+		/* TODO: unbounded recursion. This **could** be exploitable */
 	}
 
-	/* Find the continuation character '\' and strip the whitespace */
-	end = strrchr(first, '\\');
-	while (isspace(end[-1]))
-		--end;
-
-	*end = '\0'; /* Terminate the string here */
+	/* Drop the continuation character '\': to closely follow the UNIX
+	 * standard, this character **has** to be last one in the buf, with
+	 * no whitespace after it */
+	assert(is_multiline_var(value->ptr));
+	git_buf_truncate(value, value->size - 1);
 
-	len = strlen(first) + strlen(line) + 2;
-	buf = git__malloc(len);
-	if (buf == NULL) {
-		error = GIT_ENOMEM;
-		goto out;
-	}
-
-	ret = p_snprintf(buf, len, "%s %s", first, line);
-	if (ret < 0) {
-		error = git__throw(GIT_EOSERR, "Failed to parse multiline var. Failed to put together two lines. OS err: %s", strerror(errno));
-		git__free(buf);
-		goto out;
-	}
+	/* add this line to the multiline var */
+	git_buf_puts(value, line);
+	git__free(line);
 
 	/*
-	 * If we need to continue reading the next line, pretend
-	 * everything we've read up to now was in one line and call
-	 * ourselves.
+	 * If we need to continue reading the next line, let's just
+	 * keep putting stuff in the buffer
 	 */
-	if (is_multiline_var(buf)) {
-		char *final_val;
-		error = parse_multiline_variable(cfg, buf, &final_val);
-		git__free(buf);
-		buf = final_val;
-	}
-
-	*out = buf;
+	if (is_multiline_var(value->ptr))
+		return parse_multiline_variable(cfg, value);
 
- out:
-	git__free(line);
-	return error;
+	return 0;
 }
 
 static int parse_variable(diskfile_backend *cfg, char **var_name, char **var_value)
 {
-	char *tmp;
-	int error = GIT_SUCCESS;
 	const char *var_end = NULL;
 	const char *value_start = NULL;
 	char *line;
 
 	line = cfg_readline(cfg);
 	if (line == NULL)
-		return GIT_ENOMEM;
+		return -1;
 
 	strip_comments(line);
 
@@ -1260,52 +1198,39 @@ static int parse_variable(diskfile_backend *cfg, char **var_name, char **var_val
 		while (isspace(var_end[0]));
 	}
 
-	tmp = git__strndup(line, var_end - line + 1);
-	if (tmp == NULL) {
-		error = GIT_ENOMEM;
-		goto out;
-	}
+	*var_name = git__strndup(line, var_end - line + 1);
+	GITERR_CHECK_ALLOC(*var_name);
 
-	*var_name = tmp;
+	/* If there is no value, boolean true is assumed */
+	*var_value = NULL;
 
 	/*
 	 * Now, let's try to parse the value
 	 */
 	if (value_start != NULL) {
-
 		while (isspace(value_start[0]))
 			value_start++;
 
-		if (value_start[0] == '\0') {
-			*var_value = NULL;
-			goto out;
-		}
-
 		if (is_multiline_var(value_start)) {
-			error = parse_multiline_variable(cfg, value_start, var_value);
-			if (error != GIT_SUCCESS)
-			{
-				*var_value = NULL;
+			git_buf multi_value = GIT_BUF_INIT;
+			git_buf_puts(&multi_value, value_start);
+
+			if (parse_multiline_variable(cfg, &multi_value) < 0 || git_buf_oom(&multi_value)) {
 				git__free(*var_name);
+				git__free(line);
+				git_buf_free(&multi_value);
+				return -1;
 			}
-			goto out;
-		}
 
-		tmp = git__strdup(value_start);
-		if (tmp == NULL) {
-			git__free(*var_name);
-			*var_value = NULL;
-			error = GIT_ENOMEM;
-			goto out;
+			*var_value = git_buf_detach(&multi_value);
+		}
+		else if (value_start[0] != '\0') {
+			*var_value = git__strdup(value_start);
+			GITERR_CHECK_ALLOC(*var_value);
 		}
 
-		*var_value = tmp;
-	} else {
-		/* If there is no value, boolean true is assumed */
-		*var_value = NULL;
 	}
 
- out:
 	git__free(line);
-	return error;
+	return 0;
 }
diff --git a/src/errors.c b/src/errors.c
index c25fa7519..6fb7777f0 100644
--- a/src/errors.c
+++ b/src/errors.c
@@ -122,25 +122,28 @@ void giterr_set(int error_class, const char *string, ...)
 {
 	char error_str[1024];
 	va_list arglist;
-	git_error *error;
-	const char *oserr =
-		(error_class == GITERR_OS && errno != 0) ? strerror(errno) : NULL;
-
-	error = &GIT_GLOBAL->error_t;
-	free(error->message);
 
 	va_start(arglist, string);
 	p_vsnprintf(error_str, sizeof(error_str), string, arglist);
 	va_end(arglist);
 
 	/* automatically suffix strerror(errno) for GITERR_OS errors */
-	if (oserr != NULL) {
+	if (error_class == GITERR_OS) {
 		strncat(error_str, ": ", sizeof(error_str));
-		strncat(error_str, oserr, sizeof(error_str));
+		strncat(error_str, strerror(errno), sizeof(error_str));
 		errno = 0;
 	}
 
-	error->message = git__strdup(error_str);
+	giterr_set_str(error_class, error_str);
+}
+
+void giterr_set_str(int error_class, const char *string)
+{
+	git_error *error = &GIT_GLOBAL->error_t;
+
+	free(error->message);
+
+	error->message = git__strdup(string);
 	error->klass = error_class;
 
 	if (error->message == NULL) {
@@ -151,6 +154,13 @@ void giterr_set(int error_class, const char *string, ...)
 	GIT_GLOBAL->last_error = error;
 }
 
+void giterr_set_regex(const regex_t *regex, int error_code)
+{
+	char error_buf[1024];
+	regerror(error_code, regex, error_buf, sizeof(error_buf));
+	giterr_set_str(GITERR_REGEX, error_buf);
+}
+
 void giterr_clear(void)
 {
 	GIT_GLOBAL->last_error = NULL;
diff --git a/src/filebuf.c b/src/filebuf.c
index 8297b4fcf..5e206c5d8 100644
--- a/src/filebuf.c
+++ b/src/filebuf.c
@@ -14,6 +14,36 @@
 
 static const size_t WRITE_BUFFER_SIZE = (4096 * 2);
 
+enum buferr_t {
+	BUFERR_OK = 0,
+	BUFERR_WRITE,
+	BUFERR_ZLIB,
+	BUFERR_MEM
+};
+
+#define ENSURE_BUF_OK(buf) if ((buf)->last_error != BUFERR_OK) { return -1; }
+
+static int verify_last_error(git_filebuf *file)
+{
+	switch (file->last_error) {
+	case BUFERR_WRITE:
+		giterr_set(GITERR_OS, "Failed to write out file");
+		return -1;
+
+	case BUFERR_MEM:
+		giterr_set_oom();
+		return -1;
+
+	case BUFERR_ZLIB:
+		giterr_set(GITERR_ZLIB,
+			"Buffer error when writing out ZLib data");
+		return -1;
+
+	default:
+		return 0;
+	}
+}
+
 static int lock_file(git_filebuf *file, int flags)
 {
 	if (git_path_exists(file->path_lock) == true) {
@@ -100,20 +130,21 @@ GIT_INLINE(int) flush_buffer(git_filebuf *file)
 
 static int write_normal(git_filebuf *file, void *source, size_t len)
 {
-	int result = 0;
-
 	if (len > 0) {
-		result = p_write(file->fd, (void *)source, len);
+		if (p_write(file->fd, (void *)source, len) < 0) {
+			file->last_error = BUFERR_WRITE;
+			return -1;
+		}
+
 		if (file->digest)
 			git_hash_update(file->digest, source, len);
 	}
 
-	return result;
+	return 0;
 }
 
 static int write_deflate(git_filebuf *file, void *source, size_t len)
 {
-	int result = Z_OK;
 	z_stream *zs = &file->zs;
 
 	if (len > 0 || file->flush_mode == Z_FINISH) {
@@ -126,14 +157,17 @@ static int write_deflate(git_filebuf *file, void *source, size_t len)
 			zs->next_out = file->z_buf;
 			zs->avail_out = (uInt)file->buf_size;
 
-			result = deflate(zs, file->flush_mode);
-			if (result == Z_STREAM_ERROR)
-				return git__throw(GIT_ERROR, "Failed to deflate input");
+			if (deflate(zs, file->flush_mode) == Z_STREAM_ERROR) {
+				file->last_error = BUFERR_ZLIB;
+				return -1;
+			}
 
 			have = file->buf_size - (size_t)zs->avail_out;
 
-			if (p_write(file->fd, file->z_buf, have) < GIT_SUCCESS)
-				return git__throw(GIT_EOSERR, "Failed to write to file");
+			if (p_write(file->fd, file->z_buf, have) < 0) {
+				file->last_error = BUFERR_WRITE;
+				return -1;
+			}
 
 		} while (zs->avail_out == 0);
 
@@ -143,7 +177,7 @@ static int write_deflate(git_filebuf *file, void *source, size_t len)
 			git_hash_update(file->digest, source, len);
 	}
 
-	return GIT_SUCCESS;
+	return 0;
 }
 
 int git_filebuf_open(git_filebuf *file, const char *path, int flags)
@@ -161,6 +195,7 @@ int git_filebuf_open(git_filebuf *file, const char *path, int flags)
 	file->buf_size = WRITE_BUFFER_SIZE;
 	file->buf_pos = 0;
 	file->fd = -1;
+	file->last_error = BUFERR_OK;
 
 	/* Allocate the main cache buffer */
 	file->buffer = git__malloc(file->buf_size);
@@ -237,58 +272,61 @@ cleanup:
 
 int git_filebuf_hash(git_oid *oid, git_filebuf *file)
 {
-	int error;
-
 	assert(oid && file && file->digest);
 
-	if ((error = flush_buffer(file)) < GIT_SUCCESS)
-		return git__rethrow(error, "Failed to get hash for file");
+	flush_buffer(file);
+
+	if (verify_last_error(file) < 0)
+		return -1;
 
 	git_hash_final(oid, file->digest);
 	git_hash_free_ctx(file->digest);
 	file->digest = NULL;
 
-	return GIT_SUCCESS;
+	return 0;
 }
 
 int git_filebuf_commit_at(git_filebuf *file, const char *path, mode_t mode)
 {
 	git__free(file->path_original);
 	file->path_original = git__strdup(path);
-	if (file->path_original == NULL)
-		return GIT_ENOMEM;
+	GITERR_CHECK_ALLOC(file->path_original);
 
 	return git_filebuf_commit(file, mode);
 }
 
 int git_filebuf_commit(git_filebuf *file, mode_t mode)
 {
-	int error;
-
 	/* temporary files cannot be committed */
 	assert(file && file->path_original);
 
 	file->flush_mode = Z_FINISH;
-	if ((error = flush_buffer(file)) < GIT_SUCCESS)
-		goto cleanup;
+	flush_buffer(file);
+
+	if (verify_last_error(file) < 0)
+		goto on_error;
 
 	p_close(file->fd);
 	file->fd = -1;
 
 	if (p_chmod(file->path_lock, mode)) {
-		error = git__throw(GIT_EOSERR, "Failed to chmod locked file before committing");
-		goto cleanup;
+		giterr_set(GITERR_OS, "Failed to set attributes for file at '%s'", file->path_lock);
+		goto on_error;
 	}
 
 	p_unlink(file->path_original);
 
-	error = p_rename(file->path_lock, file->path_original);
+	if (p_rename(file->path_lock, file->path_original) < 0) {
+		giterr_set(GITERR_OS, "Failed to rename lockfile to '%s'", file->path_original);
+		goto on_error;
+	}
 
-cleanup:
 	git_filebuf_cleanup(file);
-	if (error < GIT_SUCCESS)
-		return git__rethrow(error, "Failed to commit locked file from buffer");
-	return GIT_SUCCESS;
+	return 0;
+
+on_error:
+	git_filebuf_cleanup(file);
+	return -1;
 }
 
 GIT_INLINE(void) add_to_cache(git_filebuf *file, const void *buf, size_t len)
@@ -299,22 +337,22 @@ GIT_INLINE(void) add_to_cache(git_filebuf *file, const void *buf, size_t len)
 
 int git_filebuf_write(git_filebuf *file, const void *buff, size_t len)
 {
-	int error;
 	const unsigned char *buf = buff;
 
+	ENSURE_BUF_OK(file);
+
 	for (;;) {
 		size_t space_left = file->buf_size - file->buf_pos;
 
 		/* cache if it's small */
 		if (space_left > len) {
 			add_to_cache(file, buf, len);
-			return GIT_SUCCESS;
+			return 0;
 		}
 
 		add_to_cache(file, buf, space_left);
-
-		if ((error = flush_buffer(file)) < GIT_SUCCESS)
-			return git__rethrow(error, "Failed to write to buffer");
+		if (flush_buffer(file) < 0)
+			return -1;
 
 		len -= space_left;
 		buf += space_left;
@@ -323,32 +361,37 @@ int git_filebuf_write(git_filebuf *file, const void *buff, size_t len)
 
 int git_filebuf_reserve(git_filebuf *file, void **buffer, size_t len)
 {
-	int error;
 	size_t space_left = file->buf_size - file->buf_pos;
 
 	*buffer = NULL;
 
-	if (len > file->buf_size)
-		return GIT_ENOMEM;
+	ENSURE_BUF_OK(file);
+
+	if (len > file->buf_size) {
+		file->last_error = BUFERR_MEM;
+		return -1;
+	}
 
 	if (space_left <= len) {
-		if ((error = flush_buffer(file)) < GIT_SUCCESS)
-			return git__rethrow(error, "Failed to reserve buffer");
+		if (flush_buffer(file) < 0)
+			return -1;
 	}
 
 	*buffer = (file->buffer + file->buf_pos);
 	file->buf_pos += len;
 
-	return GIT_SUCCESS;
+	return 0;
 }
 
 int git_filebuf_printf(git_filebuf *file, const char *format, ...)
 {
 	va_list arglist;
 	size_t space_left;
-	int len, error;
+	int len, res;
 	char *tmp_buffer;
 
+	ENSURE_BUF_OK(file);
+
 	space_left = file->buf_size - file->buf_pos;
 
 	do {
@@ -356,24 +399,28 @@ int git_filebuf_printf(git_filebuf *file, const char *format, ...)
 		len = p_vsnprintf((char *)file->buffer + file->buf_pos, space_left, format, arglist);
 		va_end(arglist);
 
-		if (len < 0)
-			return git__throw(GIT_EOSERR, "Failed to format string");
+		if (len < 0) {
+			file->last_error = BUFERR_MEM;
+			return -1;
+		}
 
 		if ((size_t)len + 1 <= space_left) {
 			file->buf_pos += len;
-			return GIT_SUCCESS;
+			return 0;
 		}
 
-		if ((error = flush_buffer(file)) < GIT_SUCCESS)
-			return git__rethrow(error, "Failed to output to buffer");
+		if (flush_buffer(file) < 0)
+			return -1;
 
 		space_left = file->buf_size - file->buf_pos;
 
 	} while ((size_t)len + 1 <= space_left);
 
 	tmp_buffer = git__malloc(len + 1);
-	if (!tmp_buffer)
-		return GIT_ENOMEM;
+	if (!tmp_buffer) {
+		file->last_error = BUFERR_MEM;
+		return -1;
+	}
 
 	va_start(arglist, format);
 	len = p_vsnprintf(tmp_buffer, len + 1, format, arglist);
@@ -381,12 +428,13 @@ int git_filebuf_printf(git_filebuf *file, const char *format, ...)
 
 	if (len < 0) {
 		git__free(tmp_buffer);
-		return git__throw(GIT_EOSERR, "Failed to format string");
+		file->last_error = BUFERR_MEM;
+		return -1;
 	}
 
-	error = git_filebuf_write(file, tmp_buffer, len);
+	res = git_filebuf_write(file, tmp_buffer, len);
 	git__free(tmp_buffer);
 
-	return error;
+	return res;
 }
 
diff --git a/src/filebuf.h b/src/filebuf.h
index 371215391..5f9d4ad9d 100644
--- a/src/filebuf.h
+++ b/src/filebuf.h
@@ -40,25 +40,35 @@ struct git_filebuf {
 
 	size_t buf_size, buf_pos;
 	git_file fd;
+	int last_error;
 };
 
 typedef struct git_filebuf git_filebuf;
 
 #define GIT_FILEBUF_INIT {0}
 
-/* The git_filebuf object lifecycle is:
+/*
+ * The git_filebuf object lifecycle is:
  * - Allocate git_filebuf, preferably using GIT_FILEBUF_INIT.
+ *
  * - Call git_filebuf_open() to initialize the filebuf for use.
+ *
  * - Make as many calls to git_filebuf_write(), git_filebuf_printf(),
- *   git_filebuf_reserve() as you like.
+ *   git_filebuf_reserve() as you like. The error codes for these
+ *   functions don't need to be checked. They are stored internally
+ *   by the file buffer.
+ *
  * - While you are writing, you may call git_filebuf_hash() to get
- *   the hash of all you have written so far.
+ *   the hash of all you have written so far. This function will
+ *   fail if any of the previous writes to the buffer failed.
+ *
  * - To close the git_filebuf, you may call git_filebuf_commit() or
  *   git_filebuf_commit_at() to save the file, or
  *   git_filebuf_cleanup() to abandon the file.  All of these will
- *   clear the git_filebuf object.
+ *   free the git_filebuf object. Likewise, all of these will fail
+ *   if any of the previous writes to the buffer failed, and set
+ *   an error code accordingly.
  */
-
 int git_filebuf_write(git_filebuf *lock, const void *buff, size_t len);
 int git_filebuf_reserve(git_filebuf *file, void **buff, size_t len);
 int git_filebuf_printf(git_filebuf *file, const char *format, ...) GIT_FORMAT_PRINTF(2, 3);
diff --git a/src/hashtable.c b/src/hashtable.c
index 73a6336c4..c081fc9a7 100644
--- a/src/hashtable.c
+++ b/src/hashtable.c
@@ -227,11 +227,11 @@ int git_hashtable_remove2(git_hashtable *self, const void *key, void **old_value
 			node->key = NULL;
 			node->value = NULL;
 			self->key_count--;
-			return GIT_SUCCESS;
+			return 0;
 		}
 	}
 
-	return git__throw(GIT_ENOTFOUND, "Entry not found in hash table");
+	return GIT_ENOTFOUND;
 }
 
 int git_hashtable_merge(git_hashtable *self, git_hashtable *other)
diff --git a/src/posix.c b/src/posix.c
index d2364d9b4..9d96d3013 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -31,10 +31,9 @@ int p_getcwd(char *buffer_out, size_t size)
 	cwd_buffer = getcwd(buffer_out, size);
 
 	if (cwd_buffer == NULL)
-		return git__throw(GIT_EOSERR, "Failed to retrieve current working directory");
+		return -1;
 
 	git_path_mkposix(buffer_out);
-
 	git_path_string_to_dir(buffer_out, size);	//Ensure the path ends with a trailing slash
 
 	return GIT_SUCCESS;
@@ -44,14 +43,13 @@ int p_rename(const char *from, const char *to)
 {
 	if (!link(from, to)) {
 		p_unlink(from);
-		return GIT_SUCCESS;
+		return 0;
 	}
 
 	if (!rename(from, to))
-		return GIT_SUCCESS;
-
-	return GIT_ERROR;
+		return 0;
 
+	return -1;
 }
 
 #endif
@@ -64,7 +62,7 @@ int p_read(git_file fd, void *buf, size_t cnt)
 		if (r < 0) {
 			if (errno == EINTR || errno == EAGAIN)
 				continue;
-			return GIT_EOSERR;
+			return -1;
 		}
 		if (!r)
 			break;
@@ -82,14 +80,14 @@ int p_write(git_file fd, const void *buf, size_t cnt)
 		if (r < 0) {
 			if (errno == EINTR || errno == EAGAIN)
 				continue;
-			return GIT_EOSERR;
+			return -1;
 		}
 		if (!r) {
 			errno = EPIPE;
-			return GIT_EOSERR;
+			return -1;
 		}
 		cnt -= r;
 		b += r;
 	}
-	return GIT_SUCCESS;
+	return 0;
 }
diff --git a/src/util.h b/src/util.h
index 01755b59e..afa3f7205 100644
--- a/src/util.h
+++ b/src/util.h
@@ -7,8 +7,6 @@
 #ifndef INCLUDE_util_h__
 #define INCLUDE_util_h__
 
-#include "git2/errors.h"
-
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
 #define bitsizeof(x) (CHAR_BIT * sizeof(x))
 #define MSB(x, bits) ((x) & (~0ULL << (bitsizeof(x) - (bits))))
@@ -24,24 +22,21 @@
 GIT_INLINE(void *) git__malloc(size_t len)
 {
 	void *ptr = malloc(len);
-	if (!ptr)
-		giterr_set(GITERR_NOMEMORY, "Out of memory. Failed to allocate %d bytes.", (int)len);
+	if (!ptr) giterr_set_oom();
 	return ptr;
 }
 
 GIT_INLINE(void *) git__calloc(size_t nelem, size_t elsize)
 {
 	void *ptr = calloc(nelem, elsize);
-	if (!ptr)
-		giterr_set(GITERR_NOMEMORY, "Out of memory. Failed to allocate %d bytes.", (int)nelem*elsize);
+	if (!ptr) giterr_set_oom();
 	return ptr;
 }
 
 GIT_INLINE(char *) git__strdup(const char *str)
 {
 	char *ptr = strdup(str);
-	if (!ptr)
-		giterr_set(GITERR_NOMEMORY, "Out of memory. Failed to duplicate string");
+	if (!ptr) giterr_set_oom();
 	return ptr;
 }
 
@@ -56,7 +51,7 @@ GIT_INLINE(char *) git__strndup(const char *str, size_t n)
 
 	ptr = (char*)malloc(length + 1);
 	if (!ptr) {
-		giterr_set(GITERR_NOMEMORY, "Out of memory. Failed to duplicate string");
+		giterr_set_oom();
 		return NULL;
 	}
 
@@ -69,8 +64,7 @@ GIT_INLINE(char *) git__strndup(const char *str, size_t n)
 GIT_INLINE(void *) git__realloc(void *ptr, size_t size)
 {
 	void *new_ptr = realloc(ptr, size);
-	if (!new_ptr)
-		giterr_set(GITERR_NOMEMORY, "Out of memory. Failed to allocate %d bytes.", (int)size);
+	if (!new_ptr) giterr_set_oom();
 	return new_ptr;
 }