Custom transport: minor cleanups

 * Move the transport registration mechanisms into a new header under
   'sys/' because this is advanced stuff.
 * Remove the 'priority' argument from the registration as it adds
   unnecessary complexity.  (Since transports cannot decline to operate,
   only the highest priority transport is ever executed.)  Users who
   require per-priority transports can implement that in their custom
   transport themselves.
 * Simplify registration further by taking a scheme (eg "http") instead
   of a prefix (eg "http://").

11 files changed, 461 insertions, 410 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d389b2cb0..93b5f20db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,12 @@ v0.21 + 1
 
 * LF -> CRLF filter now runs when * text = auto (with Git for Windows 1.9.4)
 
+* The git_transport structure definition has moved into the sys/transport.h
+  file.
+
+* The git_transport_register function no longer takes a priority and takes
+  a URL scheme name (eg "http") instead of a prefix like "http://"
+
 * The git_remote_set_transport function now sets a transport factory function,
   rather than a pre-existing transport instance.
 
diff --git a/include/git2/remote.h b/include/git2/remote.h
index c8b6ac97a..c0717fa31 100644
--- a/include/git2/remote.h
+++ b/include/git2/remote.h
@@ -419,21 +419,6 @@ GIT_EXTERN(int) git_remote_list(git_strarray *out, git_repository *repo);
 GIT_EXTERN(void) git_remote_check_cert(git_remote *remote, int check);
 
 /**
- * Sets a custom transport factory for the remote. The caller can use this
- * function to override the transport used for this remote when performing
- * network operations.
- *
- * @param remote the remote to configure
- * @param transport_cb the function to use to create a transport
- * @param payload opaque parameter passed to transport_cb
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_remote_set_transport(
-	git_remote *remote,
-	git_transport_cb transport_cb,
-	void *payload);
-
-/**
  * Argument to the completion callback which tells it which operation
  * finished.
  */
diff --git a/include/git2/sys/transport.h b/include/git2/sys/transport.h
new file mode 100644
index 000000000..62ac455d3
--- /dev/null
+++ b/include/git2/sys/transport.h
@@ -0,0 +1,354 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_sys_git_transport_h
+#define INCLUDE_sys_git_transport_h
+
+#include "git2/net.h"
+#include "git2/types.h"
+
+/**
+ * @file git2/sys/transport.h
+ * @brief Git custom transport registration interfaces and functions
+ * @defgroup git_transport Git custom transport registration
+ * @ingroup Git
+ * @{
+ */
+
+GIT_BEGIN_DECL
+
+typedef enum {
+	GIT_TRANSPORTFLAGS_NONE = 0,
+	/* If the connection is secured with SSL/TLS, the authenticity
+	 * of the server certificate should not be verified. */
+	GIT_TRANSPORTFLAGS_NO_CHECK_CERT = 1
+} git_transport_flags_t;
+
+typedef struct git_transport git_transport;
+
+struct git_transport {
+	unsigned int version;
+	/* Set progress and error callbacks */
+	int (*set_callbacks)(
+		git_transport *transport,
+		git_transport_message_cb progress_cb,
+		git_transport_message_cb error_cb,
+		void *payload);
+
+	/* Connect the transport to the remote repository, using the given
+	 * direction. */
+	int (*connect)(
+		git_transport *transport,
+		const char *url,
+		git_cred_acquire_cb cred_acquire_cb,
+		void *cred_acquire_payload,
+		int direction,
+		int flags);
+
+	/* This function may be called after a successful call to
+	 * connect(). The array returned is owned by the transport and
+	 * is guranteed until the next call of a transport function. */
+	int (*ls)(
+		const git_remote_head ***out,
+		size_t *size,
+		git_transport *transport);
+
+	/* Executes the push whose context is in the git_push object. */
+	int (*push)(git_transport *transport, git_push *push);
+
+	/* This function may be called after a successful call to connect(), when
+	 * the direction is FETCH. The function performs a negotiation to calculate
+	 * the wants list for the fetch. */
+	int (*negotiate_fetch)(
+		git_transport *transport,
+		git_repository *repo,
+		const git_remote_head * const *refs,
+		size_t count);
+
+	/* This function may be called after a successful call to negotiate_fetch(),
+	 * when the direction is FETCH. This function retrieves the pack file for
+	 * the fetch from the remote end. */
+	int (*download_pack)(
+		git_transport *transport,
+		git_repository *repo,
+		git_transfer_progress *stats,
+		git_transfer_progress_cb progress_cb,
+		void *progress_payload);
+
+	/* Checks to see if the transport is connected */
+	int (*is_connected)(git_transport *transport);
+
+	/* Reads the flags value previously passed into connect() */
+	int (*read_flags)(git_transport *transport, int *flags);
+
+	/* Cancels any outstanding transport operation */
+	void (*cancel)(git_transport *transport);
+
+	/* This function is the reverse of connect() -- it terminates the
+	 * connection to the remote end. */
+	int (*close)(git_transport *transport);
+
+	/* Frees/destructs the git_transport object. */
+	void (*free)(git_transport *transport);
+};
+
+#define GIT_TRANSPORT_VERSION 1
+#define GIT_TRANSPORT_INIT {GIT_TRANSPORT_VERSION}
+
+/**
+ * Initializes a `git_transport` with default values. Equivalent to
+ * creating an instance with GIT_TRANSPORT_INIT.
+ *
+ * @param opts the `git_transport` struct to initialize
+ * @param version Version of struct; pass `GIT_TRANSPORT_VERSION`
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_transport_init(
+	git_transport *opts,
+	unsigned int version);
+
+/**
+ * Function to use to create a transport from a URL. The transport database
+ * is scanned to find a transport that implements the scheme of the URI (i.e.
+ * git:// or http://) and a transport object is returned to the caller.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param url The URL to connect to
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_new(git_transport **out, git_remote *owner, const char *url);
+
+/**
+ * Create an ssh transport with custom git command paths
+ *
+ * This is a factory function suitable for setting as the transport
+ * callback in a remote (or for a clone in the options).
+ *
+ * The payload argument must be a strarray pointer with the paths for
+ * the `git-upload-pack` and `git-receive-pack` at index 0 and 1.
+ *
+ * @param out the resulting transport
+ * @param owner the owning remote
+ * @param payload a strarray with the paths
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_ssh_with_paths(git_transport **out, git_remote *owner, void *payload);
+
+/* Signature of a function which creates a transport */
+typedef int (*git_transport_cb)(git_transport **out, git_remote *owner, void *param);
+
+/**
+ * Add a custom transport definition, to be used in addition to the built-in
+ * set of transports that come with libgit2.
+ *
+ * The caller is responsible for synchronizing calls to git_transport_register
+ * and git_transport_unregister with other calls to the library that
+ * instantiate transports.
+ *
+ * @param prefix The scheme (ending in "://") to match, i.e. "git://"
+ * @param cb The callback used to create an instance of the transport
+ * @param param A fixed parameter to pass to cb at creation time
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_register(
+	const char *prefix,
+	git_transport_cb cb,
+	void *param);
+
+/**
+ *
+ * Unregister a custom transport definition which was previously registered
+ * with git_transport_register.
+ *
+ * @param prefix From the previous call to git_transport_register
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_unregister(
+	const char *prefix);
+
+/* Transports which come with libgit2 (match git_transport_cb). The expected
+ * value for "param" is listed in-line below. */
+
+/**
+ * Create an instance of the dummy transport.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param payload You must pass NULL for this parameter.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_dummy(
+	git_transport **out,
+	git_remote *owner,
+	/* NULL */ void *payload);
+
+/**
+ * Create an instance of the local transport.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param payload You must pass NULL for this parameter.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_local(
+	git_transport **out,
+	git_remote *owner,
+	/* NULL */ void *payload);
+
+/**
+ * Create an instance of the smart transport.
+ *
+ * @param out The newly created transport (out)
+ * @param owner The git_remote which will own this transport
+ * @param payload A pointer to a git_smart_subtransport_definition
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_transport_smart(
+	git_transport **out,
+	git_remote *owner,
+	/* (git_smart_subtransport_definition *) */ void *payload);
+
+/*
+ *** End of base transport interface ***
+ *** Begin interface for subtransports for the smart transport ***
+ */
+
+/* The smart transport knows how to speak the git protocol, but it has no
+ * knowledge of how to establish a connection between it and another endpoint,
+ * or how to move data back and forth. For this, a subtransport interface is
+ * declared, and the smart transport delegates this work to the subtransports.
+ * Three subtransports are implemented: git, http, and winhttp. (The http and
+ * winhttp transports each implement both http and https.) */
+
+/* Subtransports can either be RPC = 0 (persistent connection) or RPC = 1
+ * (request/response). The smart transport handles the differences in its own
+ * logic. The git subtransport is RPC = 0, while http and winhttp are both
+ * RPC = 1. */
+
+/* Actions that the smart transport can ask
+ * a subtransport to perform */
+typedef enum {
+	GIT_SERVICE_UPLOADPACK_LS = 1,
+	GIT_SERVICE_UPLOADPACK = 2,
+	GIT_SERVICE_RECEIVEPACK_LS = 3,
+	GIT_SERVICE_RECEIVEPACK = 4,
+} git_smart_service_t;
+
+typedef struct git_smart_subtransport git_smart_subtransport;
+typedef struct git_smart_subtransport_stream git_smart_subtransport_stream;
+
+/* A stream used by the smart transport to read and write data
+ * from a subtransport */
+struct git_smart_subtransport_stream {
+	/* The owning subtransport */
+	git_smart_subtransport *subtransport;
+
+	int (*read)(
+		git_smart_subtransport_stream *stream,
+		char *buffer,
+		size_t buf_size,
+		size_t *bytes_read);
+
+	int (*write)(
+		git_smart_subtransport_stream *stream,
+		const char *buffer,
+		size_t len);
+
+	void (*free)(
+		git_smart_subtransport_stream *stream);
+};
+
+/* An implementation of a subtransport which carries data for the
+ * smart transport */
+struct git_smart_subtransport {
+	int (* action)(
+			git_smart_subtransport_stream **out,
+			git_smart_subtransport *transport,
+			const char *url,
+			git_smart_service_t action);
+
+	/* Subtransports are guaranteed a call to close() between
+	 * calls to action(), except for the following two "natural" progressions
+	 * of actions against a constant URL.
+	 *
+	 * 1. UPLOADPACK_LS -> UPLOADPACK
+	 * 2. RECEIVEPACK_LS -> RECEIVEPACK */
+	int (*close)(git_smart_subtransport *transport);
+
+	void (*free)(git_smart_subtransport *transport);
+};
+
+/* A function which creates a new subtransport for the smart transport */
+typedef int (*git_smart_subtransport_cb)(
+	git_smart_subtransport **out,
+	git_transport* owner);
+
+typedef struct git_smart_subtransport_definition {
+	/* The function to use to create the git_smart_subtransport */
+	git_smart_subtransport_cb callback;
+
+	/* True if the protocol is stateless; false otherwise. For example,
+	 * http:// is stateless, but git:// is not. */
+	unsigned rpc;
+} git_smart_subtransport_definition;
+
+/* Smart transport subtransports that come with libgit2 */
+
+/**
+ * Create an instance of the http subtransport. This subtransport
+ * also supports https. On Win32, this subtransport may be implemented
+ * using the WinHTTP library.
+ *
+ * @param out The newly created subtransport
+ * @param owner The smart transport to own this subtransport
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_smart_subtransport_http(
+	git_smart_subtransport **out,
+	git_transport* owner);
+
+/**
+ * Create an instance of the git subtransport.
+ *
+ * @param out The newly created subtransport
+ * @param owner The smart transport to own this subtransport
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_smart_subtransport_git(
+	git_smart_subtransport **out,
+	git_transport* owner);
+
+/**
+ * Create an instance of the ssh subtransport.
+ *
+ * @param out The newly created subtransport
+ * @param owner The smart transport to own this subtransport
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_smart_subtransport_ssh(
+	git_smart_subtransport **out,
+	git_transport* owner);
+
+/**
+ * Sets a custom transport factory for the remote. The caller can use this
+ * function to override the transport used for this remote when performing
+ * network operations.
+ *
+ * @param remote the remote to configure
+ * @param transport_cb the function to use to create a transport
+ * @param payload opaque parameter passed to transport_cb
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_remote_set_transport(
+	git_remote *remote,
+	git_transport_cb transport_cb,
+	void *payload);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/include/git2/transport.h b/include/git2/transport.h
index 67939a747..66341d296 100644
--- a/include/git2/transport.h
+++ b/include/git2/transport.h
@@ -20,10 +20,6 @@
  */
 GIT_BEGIN_DECL
 
-/*
- *** Begin interface for credentials acquisition ***
- */
-
 /** Authentication type requested */
 typedef enum {
 	/* git_cred_userpass_plaintext */
@@ -227,335 +223,6 @@ typedef int (*git_cred_acquire_cb)(
 	unsigned int allowed_types,
 	void *payload);
 
-/*
- *** End interface for credentials acquisition ***
- *** Begin base transport interface ***
- */
-
-typedef enum {
-	GIT_TRANSPORTFLAGS_NONE = 0,
-	/* If the connection is secured with SSL/TLS, the authenticity
-	 * of the server certificate should not be verified. */
-	GIT_TRANSPORTFLAGS_NO_CHECK_CERT = 1
-} git_transport_flags_t;
-
-typedef int (*git_transport_message_cb)(const char *str, int len, void *data);
-
-typedef struct git_transport git_transport;
-
-struct git_transport {
-	unsigned int version;
-	/* Set progress and error callbacks */
-	int (*set_callbacks)(
-		git_transport *transport,
-		git_transport_message_cb progress_cb,
-		git_transport_message_cb error_cb,
-		void *payload);
-
-	/* Connect the transport to the remote repository, using the given
-	 * direction. */
-	int (*connect)(
-		git_transport *transport,
-		const char *url,
-		git_cred_acquire_cb cred_acquire_cb,
-		void *cred_acquire_payload,
-		int direction,
-		int flags);
-
-	/* This function may be called after a successful call to
-	 * connect(). The array returned is owned by the transport and
-	 * is guranteed until the next call of a transport function. */
-	int (*ls)(
-		const git_remote_head ***out,
-		size_t *size,
-		git_transport *transport);
-
-	/* Executes the push whose context is in the git_push object. */
-	int (*push)(git_transport *transport, git_push *push);
-
-	/* This function may be called after a successful call to connect(), when
-	 * the direction is FETCH. The function performs a negotiation to calculate
-	 * the wants list for the fetch. */
-	int (*negotiate_fetch)(
-		git_transport *transport,
-		git_repository *repo,
-		const git_remote_head * const *refs,
-		size_t count);
-
-	/* This function may be called after a successful call to negotiate_fetch(),
-	 * when the direction is FETCH. This function retrieves the pack file for
-	 * the fetch from the remote end. */
-	int (*download_pack)(
-		git_transport *transport,
-		git_repository *repo,
-		git_transfer_progress *stats,
-		git_transfer_progress_cb progress_cb,
-		void *progress_payload);
-
-	/* Checks to see if the transport is connected */
-	int (*is_connected)(git_transport *transport);
-
-	/* Reads the flags value previously passed into connect() */
-	int (*read_flags)(git_transport *transport, int *flags);
-
-	/* Cancels any outstanding transport operation */
-	void (*cancel)(git_transport *transport);
-
-	/* This function is the reverse of connect() -- it terminates the
-	 * connection to the remote end. */
-	int (*close)(git_transport *transport);
-
-	/* Frees/destructs the git_transport object. */
-	void (*free)(git_transport *transport);
-};
-
-#define GIT_TRANSPORT_VERSION 1
-#define GIT_TRANSPORT_INIT {GIT_TRANSPORT_VERSION}
-
-/**
- * Initializes a `git_transport` with default values. Equivalent to
- * creating an instance with GIT_TRANSPORT_INIT.
- *
- * @param opts the `git_transport` struct to initialize
- * @param version Version of struct; pass `GIT_TRANSPORT_VERSION`
- * @return Zero on success; -1 on failure.
- */
-GIT_EXTERN(int) git_transport_init(
-	git_transport *opts,
-	unsigned int version);
-
-/**
- * Function to use to create a transport from a URL. The transport database
- * is scanned to find a transport that implements the scheme of the URI (i.e.
- * git:// or http://) and a transport object is returned to the caller.
- *
- * @param out The newly created transport (out)
- * @param owner The git_remote which will own this transport
- * @param url The URL to connect to
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_new(git_transport **out, git_remote *owner, const char *url);
-
-/**
- * Create an ssh transport with custom git command paths
- *
- * This is a factory function suitable for setting as the transport
- * callback in a remote (or for a clone in the options).
- *
- * The payload argument must be a strarray pointer with the paths for
- * the `git-upload-pack` and `git-receive-pack` at index 0 and 1.
- *
- * @param out the resulting transport
- * @param owner the owning remote
- * @param payload a strarray with the paths
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_ssh_with_paths(git_transport **out, git_remote *owner, void *payload);
-
-/* Signature of a function which creates a transport */
-typedef int (*git_transport_cb)(git_transport **out, git_remote *owner, void *param);
-
-/**
- * Add a custom transport definition, to be used in addition to the built-in
- * set of transports that come with libgit2.
- *
- * The caller is responsible for synchronizing calls to git_transport_register
- * and git_transport_unregister with other calls to the library that
- * instantiate transports.
- *
- * @param prefix The scheme (ending in "://") to match, i.e. "git://"
- * @param priority The priority of this transport relative to others with
- *		the same prefix. Built-in transports have a priority of 1.
- * @param cb The callback used to create an instance of the transport
- * @param param A fixed parameter to pass to cb at creation time
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_register(
-	const char *prefix,
-	unsigned priority,
-	git_transport_cb cb,
-	void *param);
-
-/**
- *
- * Unregister a custom transport definition which was previously registered
- * with git_transport_register.
- *
- * @param prefix From the previous call to git_transport_register
- * @param priority From the previous call to git_transport_register
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_unregister(
-	const char *prefix,
-	unsigned priority);
-
-/* Transports which come with libgit2 (match git_transport_cb). The expected
- * value for "param" is listed in-line below. */
-
-/**
- * Create an instance of the dummy transport.
- *
- * @param out The newly created transport (out)
- * @param owner The git_remote which will own this transport
- * @param payload You must pass NULL for this parameter.
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_dummy(
-	git_transport **out,
-	git_remote *owner,
-	/* NULL */ void *payload);
-
-/**
- * Create an instance of the local transport.
- *
- * @param out The newly created transport (out)
- * @param owner The git_remote which will own this transport
- * @param payload You must pass NULL for this parameter.
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_local(
-	git_transport **out,
-	git_remote *owner,
-	/* NULL */ void *payload);
-
-/**
- * Create an instance of the smart transport.
- *
- * @param out The newly created transport (out)
- * @param owner The git_remote which will own this transport
- * @param payload A pointer to a git_smart_subtransport_definition
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_transport_smart(
-	git_transport **out,
-	git_remote *owner,
-	/* (git_smart_subtransport_definition *) */ void *payload);
-
-/*
- *** End of base transport interface ***
- *** Begin interface for subtransports for the smart transport ***
- */
-
-/* The smart transport knows how to speak the git protocol, but it has no
- * knowledge of how to establish a connection between it and another endpoint,
- * or how to move data back and forth. For this, a subtransport interface is
- * declared, and the smart transport delegates this work to the subtransports.
- * Three subtransports are implemented: git, http, and winhttp. (The http and
- * winhttp transports each implement both http and https.) */
-
-/* Subtransports can either be RPC = 0 (persistent connection) or RPC = 1
- * (request/response). The smart transport handles the differences in its own
- * logic. The git subtransport is RPC = 0, while http and winhttp are both
- * RPC = 1. */
-
-/* Actions that the smart transport can ask
- * a subtransport to perform */
-typedef enum {
-	GIT_SERVICE_UPLOADPACK_LS = 1,
-	GIT_SERVICE_UPLOADPACK = 2,
-	GIT_SERVICE_RECEIVEPACK_LS = 3,
-	GIT_SERVICE_RECEIVEPACK = 4,
-} git_smart_service_t;
-
-typedef struct git_smart_subtransport git_smart_subtransport;
-typedef struct git_smart_subtransport_stream git_smart_subtransport_stream;
-
-/* A stream used by the smart transport to read and write data
- * from a subtransport */
-struct git_smart_subtransport_stream {
-	/* The owning subtransport */
-	git_smart_subtransport *subtransport;
-
-	int (*read)(
-		git_smart_subtransport_stream *stream,
-		char *buffer,
-		size_t buf_size,
-		size_t *bytes_read);
-
-	int (*write)(
-		git_smart_subtransport_stream *stream,
-		const char *buffer,
-		size_t len);
-
-	void (*free)(
-		git_smart_subtransport_stream *stream);
-};
-
-/* An implementation of a subtransport which carries data for the
- * smart transport */
-struct git_smart_subtransport {
-	int (* action)(
-			git_smart_subtransport_stream **out,
-			git_smart_subtransport *transport,
-			const char *url,
-			git_smart_service_t action);
-
-	/* Subtransports are guaranteed a call to close() between
-	 * calls to action(), except for the following two "natural" progressions
-	 * of actions against a constant URL.
-	 *
-	 * 1. UPLOADPACK_LS -> UPLOADPACK
-	 * 2. RECEIVEPACK_LS -> RECEIVEPACK */
-	int (*close)(git_smart_subtransport *transport);
-
-	void (*free)(git_smart_subtransport *transport);
-};
-
-/* A function which creates a new subtransport for the smart transport */
-typedef int (*git_smart_subtransport_cb)(
-	git_smart_subtransport **out,
-	git_transport* owner);
-
-typedef struct git_smart_subtransport_definition {
-	/* The function to use to create the git_smart_subtransport */
-	git_smart_subtransport_cb callback;
-
-	/* True if the protocol is stateless; false otherwise. For example,
-	 * http:// is stateless, but git:// is not. */
-	unsigned rpc;
-} git_smart_subtransport_definition;
-
-/* Smart transport subtransports that come with libgit2 */
-
-/**
- * Create an instance of the http subtransport. This subtransport
- * also supports https. On Win32, this subtransport may be implemented
- * using the WinHTTP library.
- *
- * @param out The newly created subtransport
- * @param owner The smart transport to own this subtransport
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_smart_subtransport_http(
-	git_smart_subtransport **out,
-	git_transport* owner);
-
-/**
- * Create an instance of the git subtransport.
- *
- * @param out The newly created subtransport
- * @param owner The smart transport to own this subtransport
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_smart_subtransport_git(
-	git_smart_subtransport **out,
-	git_transport* owner);
-
-/**
- * Create an instance of the ssh subtransport.
- *
- * @param out The newly created subtransport
- * @param owner The smart transport to own this subtransport
- * @return 0 or an error code
- */
-GIT_EXTERN(int) git_smart_subtransport_ssh(
-	git_smart_subtransport **out,
-	git_transport* owner);
-
-/*
- *** End interface for subtransports for the smart transport ***
- */
-
 /** @} */
 GIT_END_DECL
 #endif
diff --git a/include/git2/types.h b/include/git2/types.h
index 76175b6bd..7ed1bcd4c 100644
--- a/include/git2/types.h
+++ b/include/git2/types.h
@@ -244,6 +244,16 @@ typedef struct git_transfer_progress {
 typedef int (*git_transfer_progress_cb)(const git_transfer_progress *stats, void *payload);
 
 /**
+ * Type for messages delivered by the transport.  Return a negative value
+ * to cancel the network operation.
+ *
+ * @param str The message from the transport
+ * @param len The length of the message
+ * @param payload Payload provided by the caller
+ */
+typedef int (*git_transport_message_cb)(const char *str, int len, void *payload);
+
+/**
  * Opaque structure representing a submodule.
  */
 typedef struct git_submodule git_submodule;
diff --git a/src/remote.h b/src/remote.h
index 47c4f7221..c471756b8 100644
--- a/src/remote.h
+++ b/src/remote.h
@@ -9,6 +9,7 @@
 
 #include "git2/remote.h"
 #include "git2/transport.h"
+#include "git2/sys/transport.h"
 
 #include "refspec.h"
 #include "vector.h"
diff --git a/src/transport.c b/src/transport.c
index fbcda5a53..c0311fa90 100644
--- a/src/transport.c
+++ b/src/transport.c
@@ -9,11 +9,11 @@
 #include "git2/remote.h"
 #include "git2/net.h"
 #include "git2/transport.h"
+#include "git2/sys/transport.h"
 #include "path.h"
 
 typedef struct transport_definition {
 	char *prefix;
-	unsigned priority;
 	git_transport_cb fn;
 	void *param;
 } transport_definition;
@@ -24,51 +24,54 @@ static git_smart_subtransport_definition git_subtransport_definition = { git_sma
 static git_smart_subtransport_definition ssh_subtransport_definition = { git_smart_subtransport_ssh, 0 };
 #endif
 
-static transport_definition local_transport_definition = { "file://", 1, git_transport_local, NULL };
+static transport_definition local_transport_definition = { "file://", git_transport_local, NULL };
 #ifdef GIT_SSH
-static transport_definition ssh_transport_definition = { "ssh://", 1, git_transport_smart, &ssh_subtransport_definition };
+static transport_definition ssh_transport_definition = { "ssh://", git_transport_smart, &ssh_subtransport_definition };
 #else
-static transport_definition dummy_transport_definition = { NULL, 1, git_transport_dummy, NULL };
+static transport_definition dummy_transport_definition = { NULL, git_transport_dummy, NULL };
 #endif
 
 static transport_definition transports[] = {
-	{"git://", 1, git_transport_smart, &git_subtransport_definition},
-	{"http://", 1, git_transport_smart, &http_subtransport_definition},
-	{"https://", 1, git_transport_smart, &http_subtransport_definition},
-	{"file://", 1, git_transport_local, NULL},
+	{ "git://",   git_transport_smart, &git_subtransport_definition },
+	{ "http://",  git_transport_smart, &http_subtransport_definition },
+	{ "https://", git_transport_smart, &http_subtransport_definition },
+	{ "file://",  git_transport_local, NULL },
 #ifdef GIT_SSH
-	{"ssh://", 1, git_transport_smart, &ssh_subtransport_definition},
+	{ "ssh://",   git_transport_smart, &ssh_subtransport_definition },
 #endif
-	{NULL, 0, 0}
+	{ NULL, 0, 0 }
 };
 
-static git_vector additional_transports = GIT_VECTOR_INIT;
+static git_vector custom_transports = GIT_VECTOR_INIT;
 
 #define GIT_TRANSPORT_COUNT (sizeof(transports)/sizeof(transports[0])) - 1
 
-static int transport_find_fn(const char *url, git_transport_cb *callback, void **param)
+static int transport_find_fn(
+	git_transport_cb *out,
+	const char *url,
+	void **param)
 {
 	size_t i = 0;
-	unsigned priority = 0;
-	transport_definition *definition = NULL, *definition_iter;
+	transport_definition *d, *definition = NULL;
 
-	// First, check to see if it's an obvious URL, which a URL scheme
-	for (i = 0; i < GIT_TRANSPORT_COUNT; ++i) {
-		definition_iter = &transports[i];
-
-		if (strncasecmp(url, definition_iter->prefix, strlen(definition_iter->prefix)))
-			continue;
-
-		if (definition_iter->priority > priority)
-			definition = definition_iter;
+	/* Find a user transport who wants to deal with this URI */
+	git_vector_foreach(&custom_transports, i, d) {
+		if (strncasecmp(url, d->prefix, strlen(d->prefix)) == 0) {
+			definition = d;
+			break;
+		}
 	}
 
-	git_vector_foreach(&additional_transports, i, definition_iter) {
-		if (strncasecmp(url, definition_iter->prefix, strlen(definition_iter->prefix)))
-			continue;
+	/* Find a system transport for this URI */
+	if (!definition) {
+		for (i = 0; i < GIT_TRANSPORT_COUNT; ++i) {
+			d = &transports[i];
 
-		if (definition_iter->priority > priority)
-			definition = definition_iter;
+			if (strncasecmp(url, d->prefix, strlen(d->prefix)) == 0) {
+				definition = d;
+				break;
+			}
+		}
 	}
 
 #ifdef GIT_WIN32
@@ -90,7 +93,7 @@ static int transport_find_fn(const char *url, git_transport_cb *callback, void *
 #ifdef GIT_SSH
 		definition = &ssh_transport_definition;
 #else
-        definition = &dummy_transport_definition;
+		definition = &dummy_transport_definition;
 #endif
 
 #ifndef GIT_WIN32
@@ -100,9 +103,9 @@ static int transport_find_fn(const char *url, git_transport_cb *callback, void *
 #endif
 
 	if (!definition)
-		return -1;
+		return GIT_ENOTFOUND;
 
-	*callback = definition->fn;
+	*out = definition->fn;
 	*param = definition->param;
 
 	return 0;
@@ -128,10 +131,11 @@ int git_transport_new(git_transport **out, git_remote *owner, const char *url)
 	void *param;
 	int error;
 
-	if (transport_find_fn(url, &fn, &param) < 0) {
+	if ((error = transport_find_fn(&fn, url, &param)) == GIT_ENOTFOUND) {
 		giterr_set(GITERR_NET, "Unsupported URL protocol");
 		return -1;
-	}
+	} else if (error < 0)
+		return error;
 
 	if ((error = fn(&transport, owner, param)) < 0)
 		return error;
@@ -144,59 +148,79 @@ int git_transport_new(git_transport **out, git_remote *owner, const char *url)
 }
 
 int git_transport_register(
-	const char *prefix,
-	unsigned priority,
+	const char *scheme,
 	git_transport_cb cb,
 	void *param)
 {
-	transport_definition *d;
-
-	d = git__calloc(sizeof(transport_definition), 1);
-	GITERR_CHECK_ALLOC(d);
+	git_buf prefix = GIT_BUF_INIT;
+	transport_definition *d, *definition = NULL;
+	size_t i;
+	int error = 0;
 
-	d->prefix = git__strdup(prefix);
+	assert(scheme);
+	assert(cb);
 
-	if (!d->prefix)
+	if ((error = git_buf_printf(&prefix, "%s://", scheme)) < 0)
 		goto on_error;
 
-	d->priority = priority;
-	d->fn = cb;
-	d->param = param;
+	git_vector_foreach(&custom_transports, i, d) {
+		if (strcasecmp(d->prefix, prefix.ptr) == 0) {
+			error = GIT_EEXISTS;
+			goto on_error;
+		}
+	}
+
+	definition = git__calloc(1, sizeof(transport_definition));
+	GITERR_CHECK_ALLOC(definition);
+
+	definition->prefix = git_buf_detach(&prefix);
+	definition->fn = cb;
+	definition->param = param;
 
-	if (git_vector_insert(&additional_transports, d) < 0)
+	if (git_vector_insert(&custom_transports, definition) < 0)
 		goto on_error;
 
 	return 0;
 
 on_error:
-	git__free(d->prefix);
-	git__free(d);
-	return -1;
+	git_buf_free(&prefix);
+	git__free(definition);
+	return error;
 }
 
-int git_transport_unregister(
-	const char *prefix,
-	unsigned priority)
+int git_transport_unregister(const char *scheme)
 {
+	git_buf prefix = GIT_BUF_INIT;
 	transport_definition *d;
-	unsigned i;
+	size_t i;
+	int error = 0;
+
+	assert(scheme);
 
-	git_vector_foreach(&additional_transports, i, d) {
-		if (d->priority == priority && !strcasecmp(d->prefix, prefix)) {
-			if (git_vector_remove(&additional_transports, i) < 0)
-				return -1;
+	if ((error = git_buf_printf(&prefix, "%s://", scheme)) < 0)
+		goto done;
+
+	git_vector_foreach(&custom_transports, i, d) {
+		if (strcasecmp(d->prefix, prefix.ptr) == 0) {
+			if ((error = git_vector_remove(&custom_transports, i)) < 0)
+				goto done;
 
 			git__free(d->prefix);
 			git__free(d);
 
-			if (!additional_transports.length)
-				git_vector_free(&additional_transports);
+			if (!custom_transports.length)
+				git_vector_free(&custom_transports);
 
-			return 0;
+			error = 0;
+			goto done;
 		}
 	}
 
-	return GIT_ENOTFOUND;
+	error = GIT_ENOTFOUND;
+
+done:
+	git_buf_free(&prefix);
+	return error;
 }
 
 /* from remote.h */
@@ -205,7 +229,7 @@ int git_remote_valid_url(const char *url)
 	git_transport_cb fn;
 	void *param;
 
-	return !transport_find_fn(url, &fn, &param);
+	return !transport_find_fn(&fn, url, &param);
 }
 
 int git_remote_supported_url(const char* url)
@@ -213,7 +237,7 @@ int git_remote_supported_url(const char* url)
 	git_transport_cb fn;
 	void *param;
 
-	if (transport_find_fn(url, &fn, &param) < 0)
+	if (transport_find_fn(&fn, url, &param) < 0)
 		return 0;
 
 	return fn != &git_transport_dummy;
diff --git a/src/transports/git.c b/src/transports/git.c
index 21507c1c7..e2690fe36 100644
--- a/src/transports/git.c
+++ b/src/transports/git.c
@@ -8,6 +8,7 @@
 #include "git2.h"
 #include "buffer.h"
 #include "netops.h"
+#include "git2/sys/transport.h"
 
 #define OWNING_SUBTRANSPORT(s) ((git_subtransport *)(s)->parent.subtransport)
 
diff --git a/src/transports/smart.h b/src/transports/smart.h
index f1fc29520..b29faae44 100644
--- a/src/transports/smart.h
+++ b/src/transports/smart.h
@@ -9,6 +9,7 @@
 #include "netops.h"
 #include "buffer.h"
 #include "push.h"
+#include "git2/sys/transport.h"
 
 #define GIT_SIDE_BAND_DATA     1
 #define GIT_SIDE_BAND_PROGRESS 2
diff --git a/tests/clone/transport.c b/tests/clone/transport.c
index 27568f228..46c16a241 100644
--- a/tests/clone/transport.c
+++ b/tests/clone/transport.c
@@ -2,6 +2,7 @@
 
 #include "git2/clone.h"
 #include "git2/transport.h"
+#include "git2/sys/transport.h"
 #include "fileops.h"
 
 static int custom_transport(
diff --git a/tests/structinit/structinit.c b/tests/structinit/structinit.c
index 38bedada7..def9fefc0 100644
--- a/tests/structinit/structinit.c
+++ b/tests/structinit/structinit.c
@@ -2,6 +2,7 @@
 #include <git2/sys/config.h>
 #include <git2/sys/odb_backend.h>
 #include <git2/sys/refdb_backend.h>
+#include <git2/sys/transport.h>
 
 #define STRINGIFY(s) #s