util: add functions for common file operations

Add a function to open a file for reading, writing, or appending.
In uppercase modes errors are handled as fatal, i.e. the caller doesn't
need to check for NULL. To avoid string manipulations in the callers,
the function accepts an optional directory and suffix. New files are
created with specified permissions, which will be needed for saving
keys. The O_EXCL flag is used in the writing mode to make sure a new
file is created (on filesystems that support it).

Also, add a function to rename a temporary file by changing its suffix,
and a function to remove a file.

All functions log all errors, at least as debug messages.

util.c

@@ -1089,6 +1089,152 @@ UTI_CheckDirPermissions(const char *path, mode_t perm, uid_t uid, gid_t gid)
 
 /* ================================================== */
 
+static int
+join_path(const char *basedir, const char *name, const char *suffix,
+          char *buffer, size_t length, LOG_Severity severity)
+{
+  const char *sep;
+
+  if (!basedir) {
+    basedir = "";
+    sep = "";
+  } else {
+    sep = "/";
+  }
+
+  if (!suffix)
+    suffix = "";
+
+  if (snprintf(buffer, length, "%s%s%s%s", basedir, sep, name, suffix) >= length) {
+    LOG(severity, "File path %s%s%s%s too long", basedir, sep, name, suffix);
+    return 0;
+  }
+
+  return 1;
+}
+
+/* ================================================== */
+
+FILE *
+UTI_OpenFile(const char *basedir, const char *name, const char *suffix,
+             char mode, mode_t perm)
+{
+  const char *file_mode;
+  char path[PATH_MAX];
+  LOG_Severity severity;
+  int fd, flags;
+  FILE *file;
+
+  severity = mode >= 'A' && mode <= 'Z' ? LOGS_FATAL : LOGS_ERR;
+
+  if (!join_path(basedir, name, suffix, path, sizeof (path), severity))
+    return NULL;
+
+  switch (mode) {
+    case 'r':
+    case 'R':
+      flags = O_RDONLY;
+      file_mode = "r";
+      if (severity != LOGS_FATAL)
+        severity = LOGS_DEBUG;
+      break;
+    case 'w':
+    case 'W':
+      flags = O_WRONLY | O_CREAT | O_EXCL;
+      file_mode = "w";
+      break;
+    case 'a':
+    case 'A':
+      flags = O_WRONLY | O_CREAT | O_APPEND;
+      file_mode = "a";
+      break;
+    default:
+      assert(0);
+      return NULL;
+  }
+
+try_again:
+  fd = open(path, flags, perm);
+  if (fd < 0) {
+    if (errno == EEXIST) {
+      if (unlink(path) < 0) {
+        LOG(severity, "Could not remove %s : %s", path, strerror(errno));
+        return NULL;
+      }
+      DEBUG_LOG("Removed %s", path);
+      goto try_again;
+    }
+    LOG(severity, "Could not open %s : %s", path, strerror(errno));
+    return NULL;
+  }
+
+  UTI_FdSetCloexec(fd);
+
+  file = fdopen(fd, file_mode);
+  if (!file) {
+    LOG(severity, "Could not open %s : %s", path, strerror(errno));
+    close(fd);
+    return NULL;
+  }
+
+  DEBUG_LOG("Opened %s fd=%d mode=%c", path, fd, mode);
+
+  return file;
+}
+
+/* ================================================== */
+
+int
+UTI_RenameTempFile(const char *basedir, const char *name,
+                   const char *old_suffix, const char *new_suffix)
+{
+  char old_path[PATH_MAX], new_path[PATH_MAX];
+
+  if (!join_path(basedir, name, old_suffix, old_path, sizeof (old_path), LOGS_ERR))
+    return 0;
+
+  if (!join_path(basedir, name, new_suffix, new_path, sizeof (new_path), LOGS_ERR))
+    goto error;
+
+  if (rename(old_path, new_path) < 0) {
+    LOG(LOGS_ERR, "Could not replace %s with %s : %s", new_path, old_path, strerror(errno));
+    goto error;
+  }
+
+  DEBUG_LOG("Renamed %s to %s", old_path, new_path);
+
+  return 1;
+
+error:
+  if (unlink(old_path) < 0)
+    LOG(LOGS_ERR, "Could not remove %s : %s", old_path, strerror(errno));
+
+  return 0;
+}
+
+/* ================================================== */
+
+int
+UTI_RemoveFile(const char *basedir, const char *name, const char *suffix)
+{
+  char path[PATH_MAX];
+
+  if (!join_path(basedir, name, suffix, path, sizeof (path), LOGS_ERR))
+    return 0;
+
+  if (unlink(path) < 0) {
+    LOG(errno != ENOENT ? LOGS_ERR : LOGS_DEBUG,
+        "Could not remove %s : %s", path, strerror(errno));
+    return 0;
+  }
+
+  DEBUG_LOG("Removed %s", path);
+
+  return 1;
+}
+
+/* ================================================== */
+
 void
 UTI_DropRoot(uid_t uid, gid_t gid)
 {