Ensure correct behaviour of debug_file

- Factorize re-opening of debug file according to the configuration

- Ensure that the debug file is replaced only if opening the new one is
  successful.

cfg.c

@@ -49,9 +49,7 @@ static void cfg_load_arg(cfg_t *cfg, const char *source, const char *arg) {
   } else if (strncmp(arg, "cue_prompt=", 11) == 0) {
     cfg->cue_prompt = arg + 11;
   } else if (strncmp(arg, "debug_file=", 11) == 0) {
-    const char *filename = arg + 11;
-    debug_close(cfg->debug_file);
-    cfg->debug_file = debug_open(filename);
+    cfg->debug_file = debug_replace(cfg->debug_file, arg + 11);
   } else {
     debug_dbg(cfg, "WARNING: ignored config \"%s\" from %s", arg, source);
   }
@@ -102,14 +100,17 @@ static void cfg_load_defaults(cfg_t *cfg, const char *config_path) {
 
 void cfg_init(cfg_t *cfg, int flags, int argc, const char **argv) {
   int i;
-  const char *config_path = NULL;
+  const char *config_path = NULL, *debug_file_path = NULL;
 
   memset(cfg, 0, sizeof(cfg_t));
   cfg->debug_file = DEFAULT_DEBUG_FILE;
   cfg->userpresence = -1;
   cfg->userverification = -1;
   cfg->pinverification = -1;
 
+  // Early-loaded arguments
+  // 'config=', if existing, it supplies the default configuration.
+  // 'debug' is enabled immediately to avoid blind spots.
   for (i = 0; i < argc; i++) {
     if (strcmp(argv[i], "debug") == 0) {
       cfg->debug = 1;
@@ -120,12 +121,16 @@ void cfg_init(cfg_t *cfg, int flags, int argc, const char **argv) {
       continue;
     }
     if (strncmp(argv[i], "debug_file=", 11) == 0) {
-      const char *filename = argv[i] + 11;
-      debug_close(cfg->debug_file);
-      cfg->debug_file = debug_open(filename);
+      debug_file_path = argv[i] + 11;
       continue;
     }
   }
+
+  // 'debug_file' is loaded after the loop to make sure that
+  // debug_file opening issues can be debugged too.
+  if (debug_file_path)
+    cfg->debug_file = debug_open(debug_file_path);
+
   cfg_load_defaults(cfg, config_path);
 
   for (i = 0; i < argc; i++) {

debug.c

@@ -16,21 +16,28 @@
 #define DEBUG_FMT "debug(pam_u2f): %s:%d (%s): %s%s"
 #define MSGLEN 2048
 
-FILE *debug_open(const char *filename) {
+static int debug_get_file(const char *name, FILE **out)
+{
   struct stat st;
   FILE *file;
   int fd;
 
-  if (strcmp(filename, "stdout") == 0)
-    return stdout;
-  if (strcmp(filename, "stderr") == 0)
-    return stderr;
-  if (strcmp(filename, "syslog") == 0)
-    return NULL;
+  if (strcmp(name, "stdout") == 0) {
+    *out = stdout;
+    return 0;
+  }
+  if (strcmp(name, "stderr") == 0) {
+    *out = stderr;
+    return 0;
+  }
+  if (strcmp(name, "syslog") == 0) {
+    *out = NULL;
+    return 0;
+  }
 
-  fd = open(filename, O_WRONLY | O_APPEND | O_CLOEXEC | O_NOFOLLOW | O_NOCTTY | O_CREAT);
+  fd = open(name, O_WRONLY | O_APPEND | O_CLOEXEC | O_NOFOLLOW | O_NOCTTY | O_CREAT);
   if (fd == -1 || fstat(fd, &st) != 0) {
-    D(DEFAULT_DEBUG_FILE, "Could not open %s: %s", filename, strerror(errno));
+    D(DEFAULT_DEBUG_FILE, "Could not open %s: %s", name, strerror(errno));
     goto err;
   }
 
@@ -39,19 +46,45 @@ FILE *debug_open(const char *filename) {
     goto err;
 #endif
 
-  if ((file = fdopen(fd, "a")) != NULL)
-    return file;
+  if ((file = fdopen(fd, "a")) != NULL) {
+    *out = file;
+    return 0;
+  }
 
 err:
   if (fd != -1)
     close(fd);
 
-  return DEFAULT_DEBUG_FILE; /* fallback to default */
+  return -1;
+}
+
+FILE *debug_open(const char *name) {
+  FILE *ret;
+
+  if (debug_get_file(name, &ret))
+    return DEFAULT_DEBUG_FILE;
+
+  return ret;
 }
 
 void debug_close(FILE *f) {
-  if (f != NULL && f != stdout && f != stderr)
-    fclose(f);
+  if (f == NULL || f == stdout || f == stderr)
+    return;
+
+  if (fflush(f) == EOF)
+    D(DEFAULT_DEBUG_FILE, "Could not close debug file: %s", strerror(errno));
+
+  fclose(f);
+}
+
+FILE *debug_replace(FILE *old, const char *new_name) {
+  FILE *new;
+
+  if (debug_get_file(new_name, &new))
+    return old;
+
+  debug_close(old);
+  return new;
 }
 
 static void do_log(FILE *debug_file, const char *file, int line,

debug.h

@@ -31,6 +31,8 @@
 
 FILE *debug_open(const char *);
 void debug_close(FILE *f);
+FILE *debug_replace(FILE *old, const char *new_path);
+
 void debug_fprintf(FILE *, const char *, int, const char *, const char *, ...)
   ATTRIBUTE_FORMAT(printf, 5, 6);