1 files changed, 49 insertions, 2 deletions

diff --git a/sub-process.h b/sub-process.h
index 96a2cca360..caa91a9b92 100644
--- a/sub-process.h
+++ b/sub-process.h
@@ -6,35 +6,66 @@
 #include "run-command.h"
 
 /*
- * Generic implementation of background process infrastructure.
- * See: Documentation/technical/api-sub-process.txt
+ * The sub-process API makes it possible to run background sub-processes
+ * for the entire lifetime of a Git invocation. If Git needs to communicate
+ * with an external process multiple times, then this can reduces the process
+ * invocation overhead. Git and the sub-process communicate through stdin and
+ * stdout.
+ *
+ * The sub-processes are kept in a hashmap by command name and looked up
+ * via the subprocess_find_entry function.  If an existing instance can not
+ * be found then a new process should be created and started.  When the
+ * parent git command terminates, all sub-processes are also terminated.
+ *
+ * This API is based on the run-command API.
  */
 
  /* data structures */
 
+/* Members should not be accessed directly. */
 struct subprocess_entry {
 	struct hashmap_entry ent; /* must be the first member! */
 	const char *cmd;
 	struct child_process process;
 };
 
+struct subprocess_capability {
+	const char *name;
+
+	/*
+	 * subprocess_handshake will "|=" this value to supported_capabilities
+	 * if the server reports that it supports this capability.
+	 */
+	unsigned int flag;
+};
+
 /* subprocess functions */
 
+/* Function to test two subprocess hashmap entries for equality. */
 extern int cmd2process_cmp(const void *unused_cmp_data,
 			   const struct subprocess_entry *e1,
 			   const struct subprocess_entry *e2,
 			   const void *unused_keydata);
 
+/*
+ * User-supplied function to initialize the sub-process.  This is
+ * typically used to negotiate the interface version and capabilities.
+ */
 typedef int(*subprocess_start_fn)(struct subprocess_entry *entry);
+
+/* Start a subprocess and add it to the subprocess hashmap. */
 int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, const char *cmd,
 		subprocess_start_fn startfn);
 
+/* Kill a subprocess and remove it from the subprocess hashmap. */
 void subprocess_stop(struct hashmap *hashmap, struct subprocess_entry *entry);
 
+/* Find a subprocess in the subprocess hashmap. */
 struct subprocess_entry *subprocess_find_entry(struct hashmap *hashmap, const char *cmd);
 
 /* subprocess helper functions */
 
+/* Get the underlying `struct child_process` from a subprocess. */
 static inline struct child_process *subprocess_get_child_process(
 		struct subprocess_entry *entry)
 {
@@ -42,6 +73,22 @@ static inline struct child_process *subprocess_get_child_process(
 }
 
 /*
+ * Perform the version and capability negotiation as described in the "Long
+ * Running Filter Process" section of the gitattributes documentation using the
+ * given requested versions and capabilities. The "versions" and "capabilities"
+ * parameters are arrays terminated by a 0 or blank struct.
+ *
+ * This function is typically called when a subprocess is started (as part of
+ * the "startfn" passed to subprocess_start).
+ */
+int subprocess_handshake(struct subprocess_entry *entry,
+			 const char *welcome_prefix,
+			 int *versions,
+			 int *chosen_version,
+			 struct subprocess_capability *capabilities,
+			 unsigned int *supported_capabilities);
+
+/*
  * Helper function that will read packets looking for "status=<foo>"
  * key/value pairs and return the value from the last "status" packet
  */