libstore: make temporary path prefixes optional

This is not the same thing as passing an empty string, because it
avoids the `-` separator.

Change-Id: I6a6a69646aa76953078a08c606e8f82c9ec03e8e

lix/libstore/temporary-dir.cc

@@ -5,7 +5,7 @@
 
 namespace nix {
 
-Path createTempDir(const Path & prefix, mode_t mode)
+Path createTempDir(const std::optional<Path> & prefix, mode_t mode)
 {
     return createTempSubdir(defaultTempDir(), prefix, mode);
 }

lix/libstore/temporary-dir.hh

@@ -8,7 +8,7 @@ namespace nix {
 /**
  * Create a temporary directory.
  */
-Path createTempDir(const Path & prefix = "nix", mode_t mode = 0755);
+Path createTempDir(const std::optional<Path> & prefix = "nix", mode_t mode = 0755);
 
 /**
  * Create a temporary file, returning a file handle and its path.

lix/libutil/file-system.cc

@@ -646,7 +646,7 @@ void AutoDelete::reset(const Path & p, bool recursive) {
 
 //////////////////////////////////////////////////////////////////////
 
-Path createTempSubdir(const Path & parent, const Path & prefix,
+Path createTempSubdir(const Path & parent, const std::optional<Path> & prefix,
     mode_t mode)
 {
     checkInterrupt();
@@ -670,7 +670,7 @@ Path createTempSubdir(const Path & parent, const Path & prefix,
     throw SysError("creating directory '%1%'", tmpDir);
 }
 
-Path makeTempPath(const Path & root, const Path & prefix)
+Path makeTempPath(const Path & root, const std::optional<Path> & prefix)
 {
     static thread_local std::random_device generator{};
     std::uniform_int_distribution<uint64_t> uniform_dist{};
@@ -679,7 +679,11 @@ Path makeTempPath(const Path & root, const Path & prefix)
         reinterpret_cast<const char *>(entropy),
         sizeof(entropy)
     ));
-    return fmt("%s%s-%s", root, prefix, unique);
+    if (prefix) {
+        return fmt("%s%s-%s", root, *prefix, unique);
+    } else {
+        return root + unique;
+    }
 }
 
 Path makeTempSiblingPath(const Path & path)

lix/libutil/file-system.hh

@@ -307,16 +307,16 @@ typedef std::unique_ptr<DIR, DIRDeleter> AutoCloseDir;
 /**
  * Create a temporary directory in a given parent directory.
  */
-Path createTempSubdir(const Path & parent, const Path & prefix = "nix",
+Path createTempSubdir(const Path & parent, const std::optional<Path> & prefix = "nix",
     mode_t mode = 0755);
 
 /**
  * Return temporary path constructed by appending to a root path.
  *
- * The constructed path looks like `<root><prefix>-<unique>`. To create a
+ * The constructed path looks like `<root>[<prefix>-]<unique>`. To create a
  * path nested in a directory, provide a root ending with `/`.
  */
-Path makeTempPath(const Path & root, const Path & prefix = ".tmp");
+Path makeTempPath(const Path & root, const std::optional<Path> & prefix = ".tmp");
 
 /**
  * Return temporary path in the same directory as a given path.