service: serialize information about currently executing command

Stored information will help us to resume execution after the
daemon-reload.

This commit implements following scheme,

* On serialization:
  - we count rank of the currently executing command
  - we store command type, its rank and command line arguments

* On deserialization:
  - configuration is parsed and loaded
  - we deserialize stored data, command type, rank and arguments
  - we look at the given rank in the list and if command there has same
    arguments then we restore execution at that point
  - otherwise we search respective command list and we look for command
    that has the same arguments
  - if both methods fail we do not do not resume execution at all

To better illustrate how does above scheme works, please consider
following cases (<<< denotes position where we resume execution after reload)

; Original unit file
[Service]
ExecStart=/bin/true <<<
ExecStart=/bin/false

; Swapped commands
; Second command is not going to be executed
[Service]
ExecStart=/bin/false
ExecStart=/bin/true <<<

; Commands added before
; Same commands are problematic and execution could be restarted at wrong place
[Service]
ExecStart=/bin/foo
ExecStart=/bin/bar
ExecStart=/bin/true <<<
ExecStart=/bin/false

; Commands added after
; Same commands are not an issue in this case
[Service]
ExecStart=/bin/true <<<
ExecStart=/bin/false
ExecStart=/bin/foo
ExecStart=/bin/bar

; New commands interleaved with old commands
; Some new commands will be executed while others won't
ExecStart=/bin/foo
ExecStart=/bin/true <<<
ExecStart=/bin/bar
ExecStart=/bin/false

As you can see, above scheme has some drawbacks. However, in most
cases (we assume that in most common case unit file command list is not
changed while some other command is running for the same unit) it
should cause that systemd does the right thing, which is restoring
execution exactly at the point we were before daemon-reload.

Fixes #518

src/core/service.c

@@ -45,6 +45,7 @@
 #include "service.h"
 #include "signal-util.h"
 #include "special.h"
+#include "stdio-util.h"
 #include "string-table.h"
 #include "string-util.h"
 #include "strv.h"
@@ -2140,6 +2141,80 @@ _pure_ static bool service_can_reload(Unit *u) {
         return !!s->exec_command[SERVICE_EXEC_RELOAD];
 }
 
+static unsigned service_exec_command_index(Unit *u, ServiceExecCommand id, ExecCommand *current) {
+        Service *s = SERVICE(u);
+        unsigned idx = 0;
+        ExecCommand *first, *c;
+
+        assert(s);
+
+        first = s->exec_command[id];
+
+        /* Figure out where we are in the list by walking back to the beginning */
+        for (c = current; c != first; c = c->command_prev)
+                idx++;
+
+        return idx;
+}
+
+static int service_serialize_exec_command(Unit *u, FILE *f, ExecCommand *command) {
+        Service *s = SERVICE(u);
+        ServiceExecCommand id;
+        unsigned idx;
+        const char *type;
+        char **arg;
+        _cleanup_strv_free_ char **escaped_args = NULL;
+        _cleanup_free_ char *args = NULL, *p = NULL;
+        size_t allocated = 0, length = 0;
+
+        assert(s);
+        assert(f);
+
+        if (!command)
+                return 0;
+
+        if (command == s->control_command) {
+                type = "control";
+                id = s->control_command_id;
+        } else {
+                type = "main";
+                id = SERVICE_EXEC_START;
+        }
+
+        idx = service_exec_command_index(u, id, command);
+
+        STRV_FOREACH(arg, command->argv) {
+                size_t n;
+                _cleanup_free_ char *e = NULL;
+
+                e = xescape(*arg, WHITESPACE);
+                if (!e)
+                        return -ENOMEM;
+
+                n = strlen(e);
+                if (!GREEDY_REALLOC(args, allocated, length + 1 + n + 1))
+                        return -ENOMEM;
+
+                if (length > 0)
+                        args[length++] = ' ';
+
+                memcpy(args + length, e, n);
+                length += n;
+        }
+
+        if (!GREEDY_REALLOC(args, allocated, length + 1))
+                return -ENOMEM;
+        args[length++] = 0;
+
+        p = xescape(command->path, WHITESPACE);
+        if (!p)
+                return -ENOMEM;
+
+        fprintf(f, "%s-command=%s %u %s %s\n", type, service_exec_command_to_string(id), idx, p, args);
+
+        return 0;
+}
+
 static int service_serialize(Unit *u, FILE *f, FDSet *fds) {
         Service *s = SERVICE(u);
         ServiceFDStore *fs;
@@ -2167,11 +2242,8 @@ static int service_serialize(Unit *u, FILE *f, FDSet *fds) {
         if (r < 0)
                 return r;
 
-        /* FIXME: There's a minor uncleanliness here: if there are
-         * multiple commands attached here, we will start from the
-         * first one again */
-        if (s->control_command_id >= 0)
-                unit_serialize_item(u, f, "control-command", service_exec_command_to_string(s->control_command_id));
+        service_serialize_exec_command(u, f, s->control_command);
+        service_serialize_exec_command(u, f, s->main_command);
 
         r = unit_serialize_item_fd(u, f, fds, "stdin-fd", s->stdin_fd);
         if (r < 0)
@@ -2227,6 +2299,106 @@ static int service_serialize(Unit *u, FILE *f, FDSet *fds) {
         return 0;
 }
 
+static int service_deserialize_exec_command(Unit *u, const char *key, const char *value) {
+        Service *s = SERVICE(u);
+        int r;
+        unsigned idx = 0, i;
+        bool control, found = false;
+        ServiceExecCommand id = _SERVICE_EXEC_COMMAND_INVALID;
+        ExecCommand *command = NULL;
+        _cleanup_free_ char *args = NULL, *path = NULL;
+        _cleanup_strv_free_ char **argv = NULL;
+
+        enum ExecCommandState {
+                STATE_EXEC_COMMAND_TYPE,
+                STATE_EXEC_COMMAND_INDEX,
+                STATE_EXEC_COMMAND_PATH,
+                STATE_EXEC_COMMAND_ARGS,
+                _STATE_EXEC_COMMAND_MAX,
+                _STATE_EXEC_COMMAND_INVALID = -1,
+        } state;
+
+        assert(s);
+        assert(key);
+        assert(value);
+
+        control = streq(key, "control-command");
+
+        state = STATE_EXEC_COMMAND_TYPE;
+
+        for (;;) {
+                _cleanup_free_ char *arg = NULL;
+
+                r = extract_first_word(&value, &arg, NULL, EXTRACT_CUNESCAPE);
+                if (r == 0)
+                        break;
+                else if (r < 0)
+                        return r;
+
+                switch (state) {
+                case STATE_EXEC_COMMAND_TYPE:
+                        id = service_exec_command_from_string(arg);
+                        if (id < 0)
+                                return -EINVAL;
+
+                        state = STATE_EXEC_COMMAND_INDEX;
+                        break;
+                case STATE_EXEC_COMMAND_INDEX:
+                        r = safe_atou(arg, &idx);
+                        if (r < 0)
+                                return -EINVAL;
+
+                        state = STATE_EXEC_COMMAND_PATH;
+                        break;
+                case STATE_EXEC_COMMAND_PATH:
+                        path = arg;
+                        arg = NULL;
+                        state = STATE_EXEC_COMMAND_ARGS;
+
+                        if (!path_is_absolute(path))
+                                return -EINVAL;
+                        break;
+                case STATE_EXEC_COMMAND_ARGS:
+                        r = strv_extend(&argv, arg);
+                        if (r < 0)
+                                return -ENOMEM;
+                        break;
+                default:
+                        assert_not_reached("Unknown error at deserialization of exec command");
+                        break;
+                }
+        }
+
+        if (state != STATE_EXEC_COMMAND_ARGS)
+                return -EINVAL;
+
+        /* Let's check whether exec command on given offset matches data that we just deserialized */
+        for (command = s->exec_command[id], i = 0; command; command = command->command_next, i++) {
+                if (i != idx)
+                        continue;
+
+                found = strv_equal(argv, command->argv) && streq(command->path, path);
+                break;
+        }
+
+        if (!found) {
+                /* Command at the index we serialized is different, let's look for command that exactly
+                 * matches but is on different index. If there is no such command we will not resume execution. */
+                for (command = s->exec_command[id]; command; command = command->command_next)
+                        if (strv_equal(command->argv, argv) && streq(command->path, path))
+                                break;
+        }
+
+        if (command && control)
+                s->control_command = command;
+        else if (command)
+                s->main_command = command;
+        else
+                log_unit_warning(u, "Current command vanished from the unit file, execution of the command list won't be resumed.");
+
+        return 0;
+}
+
 static int service_deserialize_item(Unit *u, const char *key, const char *value, FDSet *fds) {
         Service *s = SERVICE(u);
         int r;
@@ -2309,16 +2481,6 @@ static int service_deserialize_item(Unit *u, const char *key, const char *value,
                         s->status_text = t;
                 }
 
-        } else if (streq(key, "control-command")) {
-                ServiceExecCommand id;
-
-                id = service_exec_command_from_string(value);
-                if (id < 0)
-                        log_unit_debug(u, "Failed to parse exec-command value: %s", value);
-                else {
-                        s->control_command_id = id;
-                        s->control_command = s->exec_command[id];
-                }
         } else if (streq(key, "accept-socket")) {
                 Unit *socket;
 
@@ -2437,6 +2599,10 @@ static int service_deserialize_item(Unit *u, const char *key, const char *value,
                         s->watchdog_override_enable = true;
                         s->watchdog_override_usec = watchdog_override_usec;
                 }
+        } else if (STR_IN_SET(key, "main-command", "control-command")) {
+                r = service_deserialize_exec_command(u, key, value);
+                if (r < 0)
+                        log_unit_debug_errno(u, r, "Failed to parse serialized command \"%s\": %m", value);
         } else
                 log_unit_debug(u, "Unknown serialization key: %s", key);