sub-process: refactor handshake to common function

Refactor, into a common function, the version and capability negotiation
done when invoking a long-running process as a clean or smudge filter.
This will be useful for other Git code that needs to interact similarly
with a long-running process.

As you can see in the change to t0021, this commit changes the error
message reported when the long-running process does not introduce itself
with the expected "server"-terminated line. Originally, the error
message reports that the filter "does not support filter protocol
version 2", differentiating between the old single-file filter protocol
and the new multi-file filter protocol - I have updated it to something
more generic and useful.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sub-process.c

@@ -105,3 +105,107 @@ int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, co
 	hashmap_add(hashmap, entry);
 	return 0;
 }
+
+static int handshake_version(struct child_process *process,
+			     const char *welcome_prefix, int *versions,
+			     int *chosen_version)
+{
+	int version_scratch;
+	int i;
+	char *line;
+	const char *p;
+
+	if (!chosen_version)
+		chosen_version = &version_scratch;
+
+	if (packet_write_fmt_gently(process->in, "%s-client\n",
+				    welcome_prefix))
+		return error("Could not write client identification");
+	for (i = 0; versions[i]; i++) {
+		if (packet_write_fmt_gently(process->in, "version=%d\n",
+					    versions[i]))
+			return error("Could not write requested version");
+	}
+	if (packet_flush_gently(process->in))
+		return error("Could not write flush packet");
+
+	if (!(line = packet_read_line(process->out, NULL)) ||
+	    !skip_prefix(line, welcome_prefix, &p) ||
+	    strcmp(p, "-server"))
+		return error("Unexpected line '%s', expected %s-server",
+			     line ? line : "<flush packet>", welcome_prefix);
+	if (!(line = packet_read_line(process->out, NULL)) ||
+	    !skip_prefix(line, "version=", &p) ||
+	    strtol_i(p, 10, chosen_version))
+		return error("Unexpected line '%s', expected version",
+			     line ? line : "<flush packet>");
+	if ((line = packet_read_line(process->out, NULL)))
+		return error("Unexpected line '%s', expected flush", line);
+
+	/* Check to make sure that the version received is supported */
+	for (i = 0; versions[i]; i++) {
+		if (versions[i] == *chosen_version)
+			break;
+	}
+	if (!versions[i])
+		return error("Version %d not supported", *chosen_version);
+
+	return 0;
+}
+
+static int handshake_capabilities(struct child_process *process,
+				  struct subprocess_capability *capabilities,
+				  unsigned int *supported_capabilities)
+{
+	int i;
+	char *line;
+
+	for (i = 0; capabilities[i].name; i++) {
+		if (packet_write_fmt_gently(process->in, "capability=%s\n",
+					    capabilities[i].name))
+			return error("Could not write requested capability");
+	}
+	if (packet_flush_gently(process->in))
+		return error("Could not write flush packet");
+
+	while ((line = packet_read_line(process->out, NULL))) {
+		const char *p;
+		if (!skip_prefix(line, "capability=", &p))
+			continue;
+
+		for (i = 0;
+		     capabilities[i].name && strcmp(p, capabilities[i].name);
+		     i++)
+			;
+		if (capabilities[i].name) {
+			if (supported_capabilities)
+				*supported_capabilities |= capabilities[i].flag;
+		} else {
+			warning("external filter requested unsupported filter capability '%s'",
+				p);
+		}
+	}
+
+	return 0;
+}
+
+int subprocess_handshake(struct subprocess_entry *entry,
+			 const char *welcome_prefix,
+			 int *versions,
+			 int *chosen_version,
+			 struct subprocess_capability *capabilities,
+			 unsigned int *supported_capabilities)
+{
+	int retval;
+	struct child_process *process = &entry->process;
+
+	sigchain_push(SIGPIPE, SIG_IGN);
+
+	retval = handshake_version(process, welcome_prefix, versions,
+				   chosen_version) ||
+		 handshake_capabilities(process, capabilities,
+					supported_capabilities);
+
+	sigchain_pop(SIGPIPE);
+	return retval;
+}