feat: Add temp-dir setting

This adds a new temp-dir setting for controlling the temporary directory
without having to change the TMPDIR env var. This can be used to e.g.
use a path on a case-sensitive store on macOS for temporary files
without changing the TMPDIR var used by interactive shells or commands
invoked with `nix run`.

This also stops unsetting `TMPDIR` on darwin when the env var value
starts with `/var/folders/`, preferring instead to just do the check
when reading `TMPDIR`. This way the inherited `TMPDIR` env var is
preserved for child processes (such as interactive shells).

As a side effect this changes the behavior of `nix-build -o ''` to act
like `nix-build --no-out-link` instead of failing with an error caused
by trying to create a symlink at the cwd.

Fixes: https://git.lix.systems/lix-project/lix/issues/253
Fixes: https://git.lix.systems/lix-project/lix/issues/112
Change-Id: I9ee826323f2deca62854715a77ca7a373a948a29

src/legacy/nix-build.cc

@@ -25,6 +25,7 @@
 #include "legacy.hh"
 #include "shlex.hh"
 #include "nix-build.hh"
+#include "temporary-dir.hh"
 
 extern char * * environ __attribute__((weak)); // Man what even is this
 
@@ -55,8 +56,6 @@ static void main_nix_build(int argc, char * * argv)
     std::string script;
     std::vector<std::string> savedArgs;
 
-    AutoDelete tmpDir(createTempDir("", myName));
-
     std::string outLink = "./result";
 
     // List of environment variables kept for --pure
@@ -102,7 +101,6 @@ static void main_nix_build(int argc, char * * argv)
 
     MyArgs myArgs(myName, [&](Strings::iterator & arg, const Strings::iterator & end) {
         if (*arg == "--help") {
-            deletePath(tmpDir);
             showManPage(myName);
         }
 
@@ -113,7 +111,7 @@ static void main_nix_build(int argc, char * * argv)
             ; // obsolete
 
         else if (*arg == "--no-out-link" || *arg == "--no-link")
-            outLink = (Path) tmpDir + "/result";
+            outLink = "";
 
         else if (*arg == "--attr" || *arg == "-A")
             attrPaths.push_back(getArg(*arg, arg, end));
@@ -198,6 +196,10 @@ static void main_nix_build(int argc, char * * argv)
     if (packages && fromArgs)
         throw UsageError("'-p' and '-E' are mutually exclusive");
 
+    AutoDelete tmpDir(createTempDir("", myName));
+    if (outLink.empty())
+        outLink = (Path) tmpDir + "/result";
+
     auto store = openStore();
     auto evalStore = myArgs.evalStoreUrl ? openStore(*myArgs.evalStoreUrl) : store;
 
@@ -416,8 +418,6 @@ static void main_nix_build(int argc, char * * argv)
         // Set the environment.
         auto env = getEnv();
 
-        auto tmp = defaultTempDir();
-
         if (pure) {
             decltype(env) newEnv;
             for (auto & i : env)
@@ -428,7 +428,8 @@ static void main_nix_build(int argc, char * * argv)
             env["__ETC_PROFILE_SOURCED"] = "1";
         }
 
-        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmp;
+        // Don't use defaultTempDir() here! We want to preserve the user's TMPDIR for the shell
+        env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = getEnvNonEmpty("TMPDIR").value_or("/tmp");
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
 

src/legacy/nix-channel.cc

@@ -6,6 +6,7 @@
 #include "legacy.hh"
 #include "fetchers.hh"
 #include "eval-settings.hh" // for defexpr
+#include "temporary-dir.hh"
 #include "users.hh"
 #include "nix-channel.hh"
 

src/libcmd/cmd-profiles.cc

@@ -6,6 +6,7 @@
 #include "logging.hh"
 #include "names.hh"
 #include "store-api.hh"
+#include "temporary-dir.hh"
 #include "url-name.hh"
 
 namespace nix

src/libfetchers/git.cc

@@ -5,6 +5,7 @@
 #include "processes.hh"
 #include "tarfile.hh"
 #include "store-api.hh"
+#include "temporary-dir.hh"
 #include "url-parts.hh"
 #include "pathlocks.hh"
 #include "users.hh"

src/libfetchers/mercurial.cc

@@ -2,6 +2,7 @@
 #include "cache.hh"
 #include "processes.hh"
 #include "store-api.hh"
+#include "temporary-dir.hh"
 #include "url-parts.hh"
 #include "users.hh"
 

src/libfetchers/tarball.cc

@@ -5,6 +5,7 @@
 #include "store-api.hh"
 #include "archive.hh"
 #include "tarfile.hh"
+#include "temporary-dir.hh"
 #include "types.hh"
 #include "split.hh"
 

src/libstore/binary-cache-store.cc

@@ -8,6 +8,7 @@
 #include "remote-fs-accessor.hh"
 #include "nar-info-disk-cache.hh" // IWYU pragma: keep
 #include "nar-accessor.hh"
+#include "temporary-dir.hh"
 #include "thread-pool.hh"
 #include "signals.hh"
 #include "strings.hh"

src/libstore/build/local-derivation-goal.cc

@@ -3,6 +3,7 @@
 #include "indirect-root-store.hh"
 #include "machines.hh"
 #include "store-api.hh"
+#include "temporary-dir.hh"
 #include "worker.hh"
 #include "builtins.hh"
 #include "builtins/buildenv.hh"

src/libstore/globals.cc

@@ -468,11 +468,6 @@ void initLibStore() {
        [1] https://github.com/apple-oss-distributions/objc4/blob/01edf1705fbc3ff78a423cd21e03dfc21eb4d780/runtime/objc-initialize.mm#L614-L636
     */
     curl_global_init(CURL_GLOBAL_ALL);
-    /* On macOS, don't use the per-session TMPDIR (as set e.g. by
-       sshd). This breaks build users because they don't have access
-       to the TMPDIR, in particular in ‘nix-store --serve’. */
-    if (defaultTempDir().starts_with("/var/folders/"))
-        unsetenv("TMPDIR");
 #endif
 
     registerStoreImplementations();

src/libstore/globals.hh

@@ -615,11 +615,11 @@ public:
         )"};
 #endif
 
-    Setting<std::optional<Path>> buildDir{this, std::nullopt, "build-dir",
+    PathsSetting<std::optional<Path>> buildDir{this, std::nullopt, "build-dir",
         R"(
             The directory on the host, in which derivations' temporary build directories are created.
 
-            If not set, Nix will use the system temporary directory indicated by the `TMPDIR` environment variable.
+            If not set, Nix will use the [`temp-dir`](#conf-temp-dir) setting if set, otherwise the system temporary directory indicated by the `TMPDIR` environment variable.
             Note that builds are often performed by the Nix daemon, so its `TMPDIR` is used, and not that of the Nix command line interface.
 
             This is also the location where [`--keep-failed`](@docroot@/command-ref/opt-common.md#opt-keep-failed) leaves its files.
@@ -627,6 +627,18 @@ public:
             If Nix runs without sandbox, or if the platform does not support sandboxing with bind mounts (e.g. macOS), then the [`builder`](@docroot@/language/derivations.md#attr-builder)'s environment will contain this directory, instead of the virtual location [`sandbox-build-dir`](#conf-sandbox-build-dir).
         )"};
 
+    PathsSetting<std::optional<Path>> tempDir{this, std::nullopt, "temp-dir",
+        R"(
+            The directory on the host used as the default temporary directory.
+
+            If not set, Nix will use the system temporary directory indicated by the `TMPDIR` environment variable.
+
+            This will be used for anything that would otherwise fall back to `TMPDIR`, and the inherited `TMPDIR` value will be preserved for child processes to use.
+            If [`build-dir`](#conf-build-dir) is set, that takes precedence over this where it applies.
+
+            If set, the value must be a path that exists and is accessible to all users.
+        )"};
+
     Setting<PathSet> allowedImpureHostPrefixes{this, {}, "allowed-impure-host-deps",
         "Which prefixes to allow derivations to ask for access to (primarily for Darwin)."};
 

src/libstore/local-store.cc

@@ -2,6 +2,7 @@
 #include "globals.hh"
 #include "archive.hh"
 #include "pathlocks.hh"
+#include "temporary-dir.hh"
 #include "worker-protocol.hh"
 #include "derivations.hh"
 #include "nar-info.hh"

src/libstore/meson.build

@@ -77,6 +77,7 @@ libstore_sources = files(
   'ssh-store.cc',
   'ssh.cc',
   'store-api.cc',
+  'temporary-dir.cc',
   'uds-remote-store.cc',
   'worker-protocol.cc',
   'build/child.cc',
@@ -160,6 +161,7 @@ libstore_headers = files(
   'ssh-store.hh',
   'store-api.hh',
   'store-cast.hh',
+  'temporary-dir.hh',
   'uds-remote-store.hh',
   'worker-protocol-impl.hh',
   'worker-protocol.hh',

src/libstore/ssh.cc

@@ -4,6 +4,7 @@
 #include "finally.hh"
 #include "logging.hh"
 #include "strings.hh"
+#include "temporary-dir.hh"
 
 namespace nix {
 

src/libstore/temporary-dir.cc

@@ -0,0 +1,42 @@
+#include "temporary-dir.hh"
+
+#include "file-system.hh"
+#include "globals.hh"
+
+namespace nix {
+
+Path createTempDir(const Path & tmpRoot, const Path & prefix,
+    bool includePid, bool useGlobalCounter, mode_t mode)
+{
+    return createTempSubdir(tmpRoot.empty() ? defaultTempDir() : tmpRoot, prefix, includePid, useGlobalCounter, mode);
+}
+
+std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
+{
+    Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
+    // FIXME: use O_TMPFILE.
+    AutoCloseFD fd(mkstemp(tmpl.data()));
+    if (!fd)
+        throw SysError("creating temporary file '%s'", tmpl);
+    closeOnExec(fd.get());
+    return {std::move(fd), tmpl};
+}
+
+Path defaultTempDir()
+{
+    return settings.tempDir.get().or_else([] {
+        return getEnvNonEmpty("TMPDIR").and_then([](auto val) -> std::optional<Path> {
+#if __APPLE__
+            /* On macOS, don't use the per-session TMPDIR (as set e.g. by
+               sshd). This breaks build users because they don't have access
+               to the TMPDIR, in particular in ‘nix-store --serve’. */
+            if (val.starts_with("/var/folders/")) {
+                return std::nullopt;
+            }
+#endif
+            return val;
+        });
+    }).value_or("/tmp");
+}
+
+}

src/libstore/temporary-dir.hh

@@ -0,0 +1,24 @@
+#pragma once
+///@file
+
+#include "file-system.hh"
+
+namespace nix {
+
+/**
+ * Create a temporary directory.
+ */
+Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
+    bool includePid = true, bool useGlobalCounter = true, mode_t mode = 0755);
+
+/**
+ * Create a temporary file, returning a file handle and its path.
+ */
+std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
+
+/**
+ * Return settings.tempDir, `TMPDIR`, or the default temporary directory if unset or empty.
+ */
+Path defaultTempDir();
+
+}

src/libutil/file-system.cc

@@ -562,21 +562,17 @@ void AutoDelete::reset(const Path & p, bool recursive) {
 
 //////////////////////////////////////////////////////////////////////
 
-std::string defaultTempDir() {
-    return getEnvNonEmpty("TMPDIR").value_or("/tmp");
-}
-
-static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
+static Path tempName(PathView parent, const Path & prefix, bool includePid,
     std::atomic<unsigned int> & counter)
 {
-    tmpRoot = canonPath(tmpRoot.empty() ? defaultTempDir() : tmpRoot, true);
+    auto tmpRoot = canonPath(parent, true);
     if (includePid)
         return fmt("%1%/%2%-%3%-%4%", tmpRoot, prefix, getpid(), counter++);
     else
         return fmt("%1%/%2%-%3%", tmpRoot, prefix, counter++);
 }
 
-Path createTempDir(const Path & tmpRoot, const Path & prefix,
+Path createTempSubdir(const Path & parent, const Path & prefix,
     bool includePid, bool useGlobalCounter, mode_t mode)
 {
     static std::atomic<unsigned int> globalCounter = 0;
@@ -585,7 +581,7 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
 
     while (1) {
         checkInterrupt();
-        Path tmpDir = tempName(tmpRoot, prefix, includePid, counter);
+        Path tmpDir = tempName(parent, prefix, includePid, counter);
         if (mkdir(tmpDir.c_str(), mode) == 0) {
 #if __FreeBSD__
             /* Explicitly set the group of the directory.  This is to
@@ -606,18 +602,6 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
     }
 }
 
-
-std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
-{
-    Path tmpl(defaultTempDir() + "/" + prefix + ".XXXXXX");
-    // FIXME: use O_TMPFILE.
-    AutoCloseFD fd(mkstemp(tmpl.data()));
-    if (!fd)
-        throw SysError("creating temporary file '%s'", tmpl);
-    closeOnExec(fd.get());
-    return {std::move(fd), tmpl};
-}
-
 Path makeTempPath(const Path & root, const Path & suffix)
 {
     // start the counter at a random value to minimize issues with preexisting temp paths
@@ -721,7 +705,7 @@ void moveFile(const Path & oldName, const Path & newName)
         auto newPath = fs::path(newName);
         // For the move to be as atomic as possible, copy to a temporary
         // directory
-        fs::path temp = createTempDir(newPath.parent_path(), "rename-tmp");
+        fs::path temp = createTempSubdir(newPath.parent_path(), "rename-tmp");
         Finally removeTemp = [&]() { fs::remove(temp); };
         auto tempCopyTarget = temp / "copy-target";
         if (e.code().value() == EXDEV) {

src/libutil/file-system.hh

@@ -288,21 +288,11 @@ struct DIRDeleter
 typedef std::unique_ptr<DIR, DIRDeleter> AutoCloseDir;
 
 /**
- * Create a temporary directory.
+ * Create a temporary directory in a given parent directory.
  */
-Path createTempDir(const Path & tmpRoot = "", const Path & prefix = "nix",
+Path createTempSubdir(const Path & parent, const Path & prefix = "nix",
     bool includePid = true, bool useGlobalCounter = true, mode_t mode = 0755);
 
-/**
- * Create a temporary file, returning a file handle and its path.
- */
-std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix = "nix");
-
-/**
- * Return `TMPDIR`, or the default temporary directory if unset or empty.
- */
-Path defaultTempDir();
-
 /**
  * Return temporary path constructed by appending a suffix to a root path.
  *

src/nix/develop.cc

@@ -7,6 +7,7 @@
 #include "outputs-spec.hh"
 #include "derivations.hh"
 #include "run.hh"
+#include "temporary-dir.hh"
 
 #include <iterator>
 #include <memory>

src/nix/prefetch.cc

@@ -8,6 +8,7 @@
 #include "attr-path.hh"
 #include "eval-inline.hh" // IWYU pragma: keep
 #include "legacy.hh"
+#include "temporary-dir.hh"
 #include "terminal.hh"
 #include "prefetch-command.hh"
 

src/nix/run.cc

@@ -13,6 +13,7 @@
 #include "current-process.hh"
 
 #if __linux__
+#include "temporary-dir.hh"
 #include <sys/mount.h>
 #endif