logmsg_reencode(): warn when iconv() fails

If the user asks for a pretty-printed commit to be converted (either
explicitly with --encoding=foo, or implicitly because the commit is
non-utf8 and we want to convert it), we pass it through iconv(). If that
fails, we fall back to showing the input verbatim, but don't tell the
user that the output may be bogus.

Let's add a warning to do so, along with a mention in the documentation
for --encoding. Two things to note about the implementation:

  - we could produce the warning closer to the call to iconv() in
    reencode_string_len(), which would let us relay the value of errno.
    But this is not actually very helpful. reencode_string_len() does
    not know we are operating on a commit, and indeed does not know that
    the caller won't produce an error of its own. And the errno values
    from iconv() are seldom helpful (iconv_open() only ever produces
    EINVAL; perhaps EILSEQ from iconv() might be illuminating, but it
    can also return EINVAL for incomplete sequences).

  - if the reason for the failure is that the output charset is not
    supported, then the user will see this warning for every commit we
    try to display. That might be ugly and overwhelming, but on the
    other hand it is making it clear that every one of them has not been
    converted (and the likely outcome anyway is to re-try the command
    with a supported output encoding).

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

1 files changed, 5 insertions, 1 deletions

diff --git a/pretty.c b/pretty.c
index 9631529c10..73b5ead509 100644
--- a/pretty.c
+++ b/pretty.c
@@ -671,7 +671,11 @@ const char *repo_logmsg_reencode(struct repository *r,
 	 * If the re-encoding failed, out might be NULL here; in that
 	 * case we just return the commit message verbatim.
 	 */
-	return out ? out : msg;
+	if (!out) {
+		warning("unable to reencode commit to '%s'", output_encoding);
+		return msg;
+	}
+	return out;
 }
 
 static int mailmap_name(const char **email, size_t *email_len,