core: rework serialization

Let's be more careful with what we serialize: let's ensure we never
serialize strings that are longer than LONG_LINE_MAX, so that we know we
can read them back with read_line(…, LONG_LINE_MAX, …) safely.

In order to implement this all serialization functions are move to
serialize.[ch], and internally will do line size checks. We'd rather
skip a serialization line (with a loud warning) than write an overly
long line out. Of course, this is just a second level protection, after
all the data we serialize shouldn't be this long in the first place.

While we are at it also clean up logging: while serializing make sure to
always log about errors immediately. Also, (void)ify all calls we don't
expect errors in (or catch errors as part of the general
fflush_and_check() at the end.

src/basic/exec-util.c

@@ -9,6 +9,7 @@
 
 #include "alloc-util.h"
 #include "conf-files.h"
+#include "def.h"
 #include "env-util.h"
 #include "exec-util.h"
 #include "fd-util.h"
@@ -16,6 +17,7 @@
 #include "hashmap.h"
 #include "macro.h"
 #include "process-util.h"
+#include "serialize.h"
 #include "set.h"
 #include "signal-util.h"
 #include "stat-util.h"
@@ -282,7 +284,7 @@ static int gather_environment_collect(int fd, void *arg) {
                 return -errno;
         }
 
-        r = serialize_environment(f, *env);
+        r = serialize_strv(f, "env", *env);
         if (r < 0)
                 return r;
 
@@ -294,29 +296,47 @@ static int gather_environment_collect(int fd, void *arg) {
 }
 
 static int gather_environment_consume(int fd, void *arg) {
-        char ***env = arg;
         _cleanup_fclose_ FILE *f = NULL;
-        char line[LINE_MAX];
-        int r = 0, k;
+        char ***env = arg;
+        int r = 0;
 
         /* Read a series of env=cescape(VAR=value) assignments from fd into env. */
 
         assert(env);
 
-        f = fdopen(fd, "r");
+        f = fdopen(fd, "re");
         if (!f) {
                 safe_close(fd);
                 return -errno;
         }
 
-        FOREACH_LINE(line, f, return -EIO) {
-                truncate_nl(line);
+        for (;;) {
+                _cleanup_free_ char *line = NULL;
+                const char *v;
+                int k;
 
-                k = deserialize_environment(env, line);
+                k = read_line(f, LONG_LINE_MAX, &line);
                 if (k < 0)
-                        log_error_errno(k, "Invalid line \"%s\": %m", line);
-                if (k < 0 && r == 0)
-                        r = k;
+                        return k;
+                if (k == 0)
+                        break;
+
+                v = startswith(line, "env=");
+                if (!v) {
+                        log_debug("Serialization line \"%s\" unexpectedly didn't start with \"env=\".", line);
+                        if (r == 0)
+                                r = -EINVAL;
+
+                        continue;
+                }
+
+                k = deserialize_environment(v, env);
+                if (k < 0) {
+                        log_debug_errno(k, "Invalid serialization line \"%s\": %m", line);
+
+                        if (r == 0)
+                                r = k;
+                }
         }
 
         return r;