Allow frontends to bidirectionally communicate with fast-import

The existing checkpoint command is very useful to force fast-import
to dump the branches out to disk so that standard Git tools can
access them and the objects they refer to.  However there was not a
way to know when fast-import had finished executing the checkpoint
and it was safe to read those refs.

The progress command can be used to make fast-import output any
message of the frontend's choosing to standard out.  The frontend
can scan for these messages using select() or poll() to monitor a
pipe connected to the standard output of fast-import.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/fast-import.c b/fast-import.c
index 4bc7f81bc..429ab491b 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -8,6 +8,7 @@ Format of STDIN stream:
         | new_tag
         | reset_branch
         | checkpoint
+        | progress
         ;
 
   new_blob ::= 'blob' lf
@@ -53,6 +54,9 @@ Format of STDIN stream:
   checkpoint ::= 'checkpoint' lf
     lf?;
 
+  progress ::= 'progress' sp not_lf* lf
+    lf?;
+
      # note: the first idnum in a stream should be 1 and subsequent
      # idnums should not have gaps between values as this will cause
      # the stream parser to reserve space for the gapped values.  An
@@ -2125,6 +2129,14 @@ static void cmd_checkpoint(void)
 	skip_optional_lf();
 }
 
+static void cmd_progress(void)
+{
+	fwrite(command_buf.buf, 1, command_buf.len - 1, stdout);
+	fputc('\n', stdout);
+	fflush(stdout);
+	skip_optional_lf();
+}
+
 static void import_marks(const char *input_file)
 {
 	char line[512];
@@ -2235,6 +2247,8 @@ int main(int argc, const char **argv)
 			cmd_reset_branch();
 		else if (!strcmp("checkpoint", command_buf.buf))
 			cmd_checkpoint();
+		else if (!prefixcmp(command_buf.buf, "progress "))
+			cmd_progress();
 		else
 			die("Unsupported command: %s", command_buf.buf);
 	}