[RFC PATCH] commit: Warn about encodings unsupported by iconv.

[Alexander Gavrilov <angavrilov@gmail.com>]

Currently git-commit and git-commit-tree silently allow
usage of encodings that are unknown to iconv. This may
confuse the user, who then won't be able to use automatic
encoding conversion in git-log and friends without any
immediately obvious reason. Note that the difference
between a supported and an unsupported name may be as
small as CP1251 vs CP-1251, or Shift-JIS vs ShiftJIS.

This commit adds a check for such cases, which produces
a warning similar to the one issued when a message claims
to be utf-8, but actually is not.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
---

	I wonder how common such situation may actually be, and 
	whether gitk & git-gui (or core git itself) should explicitly
	provide some way to deal with it in old commits. I personally
	hit it during experimenting, and wrongly concluded that gitk
	does not support using multiple encodings in one repository.
	
	Current gitk implementation generally allows working around
	it by setting i18n.commitencoding to a valid name of the
	encoding used in the mislabeled commits. However, the
	downside is that if the selected encoding cannot represent
	some characters of an otherwise completely valid commit,
	they come out as garbage. Always using --encoding=utf-8
	from GUI and relying on conversion done by git-log should
	fix this case, but it breaks the workaround.
	
	-- Alexander

 builtin-commit-tree.c |   25 +++++++++++++++++++++++--
 1 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/builtin-commit-tree.c b/builtin-commit-tree.c
index 0453425..7f325a3 100644
--- a/builtin-commit-tree.c
+++ b/builtin-commit-tree.c
@@ -45,6 +45,28 @@ static const char commit_utf8_warn[] =
 "You may want to amend it after fixing the message, or set the config\n"
 "variable i18n.commitencoding to the encoding your project uses.\n";
 
+static const char commit_bad_encoding_warn[] =
+"Warning: commit encoding '%s' is not supported.\n"
+"You may want to change the value of the i18n.commitencoding config\n"
+"variable, and redo the commit. Use 'iconv --list' to see the list of\n"
+"available encoding names.\n";
+
+static void verify_commit_encoding(const char *text, const char *encoding)
+{
+	if (is_encoding_utf8(encoding)) {
+		if (!is_utf8(text))
+			fprintf(stderr, commit_utf8_warn);
+	}
+#ifndef NO_ICONV
+	else {
+		char *conv = reencode_string("", "utf-8", encoding);
+		if (!conv)
+			fprintf(stderr, commit_bad_encoding_warn, encoding);
+		free(conv);
+	}
+#endif
+}
+
 int commit_tree(const char *msg, unsigned char *tree,
 		struct commit_list *parents, unsigned char *ret,
 		const char *author)
@@ -87,8 +109,7 @@ int commit_tree(const char *msg, unsigned char *tree,
 	strbuf_addstr(&buffer, msg);
 
 	/* And check the encoding */
-	if (encoding_is_utf8 && !is_utf8(buffer.buf))
-		fprintf(stderr, commit_utf8_warn);
+	verify_commit_encoding(buffer.buf, git_commit_encoding);
 
 	result = write_sha1_file(buffer.buf, buffer.len, commit_type, ret);
 	strbuf_release(&buffer);
-- 
1.6.0.20.g6148bc