fix: Treat empty TMPDIR as unset

Fixes an instance of

    nix: src/libutil/util.cc:139: nix::Path nix::canonPath(PathView, bool): Assertion `path != ""' failed.

... which I've been getting in one of my shells for some reason.
I have yet to find out why TMPDIR was empty, but it's no reason for
Nix to break.

(cherry picked from commit c3fb2aa1f9)

src/libstore/globals.cc

@@ -408,7 +408,7 @@ void initLibStore() {
        sshd). This breaks build users because they don't have access
        to the TMPDIR, in particular in ‘nix-store --serve’. */
 #if __APPLE__
-    if (hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
+    if (hasPrefix(defaultTempDir(), "/var/folders/"))
         unsetenv("TMPDIR");
 #endif
 

src/libutil/file-system.hh

@@ -0,0 +1,17 @@
+#pragma once
+/**
+ * @file
+ *
+ * Utiltities for working with the file sytem and file paths.
+ */
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Return `TMPDIR`, or the default temporary directory if unset or empty.
+ */
+Path defaultTempDir();
+
+}

src/libutil/filesystem.cc

@@ -10,10 +10,14 @@ namespace fs = std::filesystem;
 
 namespace nix {
 
+std::string defaultTempDir() {
+    return getEnvNonEmpty("TMPDIR").value_or("/tmp");
+}
+
 static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
     std::atomic<unsigned int> & counter)
 {
-    tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
+    tmpRoot = canonPath(tmpRoot.empty() ? defaultTempDir() : tmpRoot, true);
     if (includePid)
         return fmt("%1%/%2%-%3%-%4%", tmpRoot, prefix, getpid(), counter++);
     else
@@ -53,7 +57,7 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 
 std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
 {
-    Path tmpl(getEnv("TMPDIR").value_or("/tmp") + "/" + prefix + ".XXXXXX");
+    Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
     // Strictly speaking, this is UB, but who cares...
     // FIXME: use O_TMPFILE.
     AutoCloseFD fd(mkstemp((char *) tmpl.c_str()));

src/nix-build/nix-build.cc

@@ -458,7 +458,7 @@ static void main_nix_build(int argc, char * * argv)
         auto env = getEnv();
 
         auto tmp = getEnv("TMPDIR");
-        if (!tmp) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");
+        if (!tmp || tmp->empty()) tmp = getEnv("XDG_RUNTIME_DIR").value_or("/tmp");
 
         if (pure) {
             decltype(env) newEnv;