diff: clarify textconv interface

The memory allocation scheme for the textconv interface is a
bit tricky, and not well documented. It was originally
designed as an internal part of diff.c (matching
fill_mmfile), but gradually was made public.

Refactoring it is difficult, but we can at least improve the
situation by documenting the intended flow and enforcing it
with an in-code assertion.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -4996,7 +4996,7 @@ size_t fill_textconv(struct userdiff_driver *driver,
 {
 	size_t size;
 
-	if (!driver || !driver->textconv) {
+	if (!driver) {
 		if (!DIFF_FILE_VALID(df)) {
 			*outbuf = "";
 			return 0;
@@ -5007,6 +5007,9 @@ size_t fill_textconv(struct userdiff_driver *driver,
 		return df->size;
 	}
 
+	if (!driver->textconv)
+		die("BUG: fill_textconv called with non-textconv driver");
+
 	if (driver->textconv_cache && df->sha1_valid) {
 		*outbuf = notes_cache_get(driver->textconv_cache, df->sha1,
 					  &size);

diff.h

@@ -342,10 +342,26 @@ extern void diff_no_index(struct rev_info *, int, const char **, const char *);
 
 extern int index_differs_from(const char *def, int diff_flags);
 
+/*
+ * Fill the contents of the filespec "df", respecting any textconv defined by
+ * its userdiff driver.  The "driver" parameter must come from a
+ * previous call to get_textconv(), and therefore should either be NULL or have
+ * textconv enabled.
+ *
+ * Note that the memory ownership of the resulting buffer depends on whether
+ * the driver field is NULL. If it is, then the memory belongs to the filespec
+ * struct. If it is non-NULL, then "outbuf" points to a newly allocated buffer
+ * that should be freed by the caller.
+ */
 extern size_t fill_textconv(struct userdiff_driver *driver,
 			    struct diff_filespec *df,
 			    char **outbuf);
 
+/*
+ * Look up the userdiff driver for the given filespec, and return it if
+ * and only if it has textconv enabled (otherwise return NULL). The result
+ * can be passed to fill_textconv().
+ */
 extern struct userdiff_driver *get_textconv(struct diff_filespec *one);
 
 extern int parse_rename_score(const char **cp_p);

userdiff.h

@@ -23,6 +23,10 @@ int userdiff_config(const char *k, const char *v);
 struct userdiff_driver *userdiff_find_by_name(const char *name);
 struct userdiff_driver *userdiff_find_by_path(const char *path);
 
+/*
+ * Initialize any textconv-related fields in the driver and return it, or NULL
+ * if it does not have textconv enabled at all.
+ */
 struct userdiff_driver *userdiff_get_textconv(struct userdiff_driver *driver);
 
 #endif /* USERDIFF */