fast-import: Allow cat-blob requests at arbitrary points in stream

The new rule: a "cat-blob" can be inserted wherever a comment is
allowed, which means at the start of any line except in the middle of
a "data" command.

This saves frontends from having to loop over everything they want to
commit in the next commit and cat-ing the necessary objects in
advance.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -912,6 +912,10 @@ output uses the same format as `git cat-file --batch`:
 	<contents> LF
 ====
 
+This command can be used anywhere in the stream that comments are
+accepted.  In particular, the `cat-blob` command can be used in the
+middle of a commit but not in the middle of a `data` command.
+
 `feature`
 ~~~~~~~~~
 Require that fast-import supports the specified feature, or abort if

fast-import.c

@@ -55,8 +55,6 @@ Format of STDIN stream:
     ('from' sp committish lf)?
     lf?;
 
-  cat_blob ::= 'cat-blob' sp (hexsha1 | idnum) lf;
-
   checkpoint ::= 'checkpoint' lf
     lf?;
 
@@ -134,14 +132,17 @@ Format of STDIN stream:
   ts    ::= # time since the epoch in seconds, ascii base10 notation;
   tz    ::= # GIT style timezone;
 
-     # note: comments may appear anywhere in the input, except
-     # within a data command.  Any form of the data command
-     # always escapes the related input from comment processing.
+     # note: comments and cat requests may appear anywhere
+     # in the input, except within a data command.  Any form
+     # of the data command always escapes the related input
+     # from comment processing.
      #
      # In case it is not clear, the '#' that starts the comment
      # must be the first character on that line (an lf
      # preceded it).
      #
+  cat_blob ::= 'cat-blob' sp (hexsha1 | idnum) lf;
+
   comment ::= '#' not_lf* lf;
   not_lf  ::= # Any byte that is not ASCII newline (LF);
 */
@@ -367,6 +368,7 @@ static int seen_data_command;
 static int cat_blob_fd = STDOUT_FILENO;
 
 static void parse_argv(void);
+static void parse_cat_blob(void);
 
 static void write_branch_report(FILE *rpt, struct branch *b)
 {
@@ -1785,7 +1787,6 @@ static void read_marks(void)
 	fclose(f);
 }
 
-
 static int read_next_command(void)
 {
 	static int stdin_eof = 0;
@@ -1795,7 +1796,7 @@ static int read_next_command(void)
 		return EOF;
 	}
 
-	do {
+	for (;;) {
 		if (unread_command_buf) {
 			unread_command_buf = 0;
 		} else {
@@ -1828,9 +1829,14 @@ static int read_next_command(void)
 			rc->prev->next = rc;
 			cmd_tail = rc;
 		}
-	} while (command_buf.buf[0] == '#');
-
+		if (!prefixcmp(command_buf.buf, "cat-blob ")) {
+			parse_cat_blob();
+			continue;
+		}
+		if (command_buf.buf[0] == '#')
+			continue;
 		return 0;
+	}
 }
 
 static void skip_optional_lf(void)
@@ -3066,8 +3072,6 @@ int main(int argc, const char **argv)
 			parse_new_tag();
 		else if (!prefixcmp(command_buf.buf, "reset "))
 			parse_reset_branch();
-		else if (!prefixcmp(command_buf.buf, "cat-blob "))
-			parse_cat_blob();
 		else if (!strcmp("checkpoint", command_buf.buf))
 			parse_checkpoint();
 		else if (!prefixcmp(command_buf.buf, "progress "))

t/t9300-fast-import.sh

@@ -1823,6 +1823,72 @@ test_expect_success PIPE 'R: copy using cat-file' '
 	test_cmp big actual
 '
 
+test_expect_success PIPE 'R: print blob mid-commit' '
+	rm -f blobs &&
+	echo "A blob from _before_ the commit." >expect &&
+	mkfifo blobs &&
+	(
+		exec 3<blobs &&
+		cat <<-EOF &&
+		feature cat-blob
+		blob
+		mark :1
+		data <<BLOB
+		A blob from _before_ the commit.
+		BLOB
+		commit refs/heads/temporary
+		committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+		data <<COMMIT
+		Empty commit
+		COMMIT
+		cat-blob :1
+		EOF
+
+		read blob_id type size <&3 &&
+		dd if=/dev/stdin of=actual bs=$size count=1 <&3 &&
+		read newline <&3 &&
+
+		echo
+	) |
+	git fast-import --cat-blob-fd=3 3>blobs &&
+	test_cmp expect actual
+'
+
+test_expect_success PIPE 'R: print staged blob within commit' '
+	rm -f blobs &&
+	echo "A blob from _within_ the commit." >expect &&
+	mkfifo blobs &&
+	(
+		exec 3<blobs &&
+		cat <<-EOF &&
+		feature cat-blob
+		commit refs/heads/within
+		committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+		data <<COMMIT
+		Empty commit
+		COMMIT
+		M 644 inline within
+		data <<BLOB
+		A blob from _within_ the commit.
+		BLOB
+		EOF
+
+		to_get=$(
+			echo "A blob from _within_ the commit." |
+			git hash-object --stdin
+		) &&
+		echo "cat-blob $to_get" &&
+
+		read blob_id type size <&3 &&
+		dd if=/dev/stdin of=actual bs=$size count=1 <&3 &&
+		read newline <&3 &&
+
+		echo deleteall
+	) |
+	git fast-import --cat-blob-fd=3 3>blobs &&
+	test_cmp expect actual
+'
+
 cat >input << EOF
 option git quiet
 blob