refs.c: optimize check_refname_component()dt/refs-check-refname-component-optim

In a repository with many refs, check_refname_component can be a major
contributor to the runtime of some git commands. One such command is

git rev-parse HEAD

Timings for one particular repo, with about 60k refs, almost all
packed, are:

Old: 35 ms
New: 29 ms

Many other commands which read refs are also sped up.

Signed-off-by: David Turner <dturner@twitter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 44 insertions, 29 deletions

diff --git a/refs.c b/refs.c
index 728a761648..637a93d695 100644
--- a/refs.c
+++ b/refs.c
@@ -6,8 +6,29 @@
 #include "string-list.h"
 
 /*
- * Make sure "ref" is something reasonable to have under ".git/refs/";
- * We do not like it if:
+ * How to handle various characters in refnames:
+ * 0: An acceptable character for refs
+ * 1: End-of-component
+ * 2: ., look for a preceding . to reject .. in refs
+ * 3: {, look for a preceding @ to reject @{ in refs
+ * 4: A bad character: ASCII control characters, "~", "^", ":" or SP
+ */
+static unsigned char refname_disposition[256] = {
+	1, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+	4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+	4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 2, 1,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 4,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 4, 0, 4, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 4, 4
+};
+
+/*
+ * Try to read one refname component from the front of refname.
+ * Return the length of the component found, or -1 if the component is
+ * not legal.  It is legal if it is something reasonable to have under
+ * ".git/refs/"; We do not like it if:
  *
  * - any path component of it begins with ".", or
  * - it has double dots "..", or
@@ -16,41 +37,31 @@
  * - it ends with ".lock"
  * - it contains a "\" (backslash)
  */
-
-/* Return true iff ch is not allowed in reference names. */
-static inline int bad_ref_char(int ch)
-{
-	if (((unsigned) ch) <= ' ' || ch == 0x7f ||
-	    ch == '~' || ch == '^' || ch == ':' || ch == '\\')
-		return 1;
-	/* 2.13 Pattern Matching Notation */
-	if (ch == '*' || ch == '?' || ch == '[') /* Unsupported */
-		return 1;
-	return 0;
-}
-
-/*
- * Try to read one refname component from the front of refname.  Return
- * the length of the component found, or -1 if the component is not
- * legal.
- */
 static int check_refname_component(const char *refname, int flags)
 {
 	const char *cp;
 	char last = '\0';
 
 	for (cp = refname; ; cp++) {
-		char ch = *cp;
-		if (ch == '\0' || ch == '/')
+		int ch = *cp & 255;
+		unsigned char disp = refname_disposition[ch];
+		switch (disp) {
+		case 1:
+			goto out;
+		case 2:
+			if (last == '.')
+				return -1; /* Refname contains "..". */
+			break;
+		case 3:
+			if (last == '@')
+				return -1; /* Refname contains "@{". */
 			break;
-		if (bad_ref_char(ch))
-			return -1; /* Illegal character in refname. */
-		if (last == '.' && ch == '.')
-			return -1; /* Refname contains "..". */
-		if (last == '@' && ch == '{')
-			return -1; /* Refname contains "@{". */
+		case 4:
+			return -1;
+		}
 		last = ch;
 	}
+out:
 	if (cp == refname)
 		return 0; /* Component has zero length. */
 	if (refname[0] == '.') {
diff --git a/t/t5511-refspec.sh b/t/t5511-refspec.sh
index c28932216b..de6db86ccf 100755
--- a/t/t5511-refspec.sh
+++ b/t/t5511-refspec.sh
@@ -5,7 +5,6 @@ test_description='refspec parsing'
 . ./test-lib.sh
 
 test_refspec () {
-
 	kind=$1 refspec=$2 expect=$3
 	git config remote.frotz.url "." &&
 	git config --remove-section remote.frotz &&
@@ -84,4 +83,9 @@ test_refspec push 'refs/heads/*/*/for-linus:refs/remotes/mine/*' invalid
 test_refspec fetch 'refs/heads/*/for-linus:refs/remotes/mine/*'
 test_refspec push 'refs/heads/*/for-linus:refs/remotes/mine/*'
 
+good=$(printf '\303\204')
+test_refspec fetch "refs/heads/${good}"
+bad=$(printf '\011tab')
+test_refspec fetch "refs/heads/${bad}"				invalid
+
 test_done