fetchMercurial: set HGPLAIN when invoking hg

Without setting HGPLAIN, the user's environment leaks into
hg invocations, which means that the output may not be in the
expected format.

HGPLAIN is the Mercurial-recommended solution for this in that
it's intended for uses by scripts and programs which are looking
to parse Mercurial's output in a consistent manner.

src/libfetchers/mercurial.cc

@@ -11,6 +11,36 @@ using namespace std::string_literals;
 
 namespace nix::fetchers {
 
+namespace {
+
+RunOptions hgOptions(const Strings & args) {
+	RunOptions opts("hg", args);
+	opts.searchPath = true;
+
+	auto env = getEnv();
+	// Set HGPLAIN: this means we get consistent output from hg and avoids leakage from a user or system .hgrc.
+	env["HGPLAIN"] = "";
+	opts.environment = env;
+
+	return opts;
+}
+
+// runProgram wrapper that uses hgOptions instead of stock RunOptions.
+string runHg(const Strings & args, const std::optional<std::string> & input = {})
+{
+	RunOptions opts = hgOptions(args);
+	opts.input = input;
+
+	auto res = runProgram(opts);
+
+	if (!statusOk(res.first))
+		throw ExecError(res.first, fmt("hg %1%", statusToString(res.first)));
+
+	return res.second;
+}
+
+}
+
 struct MercurialInputScheme : InputScheme
 {
     std::optional<Input> inputFromURL(const ParsedURL & url) override
@@ -100,11 +130,11 @@ struct MercurialInputScheme : InputScheme
         assert(sourcePath);
 
         // FIXME: shut up if file is already tracked.
-        runProgram("hg", true,
+        runHg(
             { "add", *sourcePath + "/" + std::string(file) });
 
         if (commitMsg)
-            runProgram("hg", true,
+            runHg(
                 { "commit", *sourcePath + "/" + std::string(file), "-m", *commitMsg });
     }
 
@@ -130,7 +160,7 @@ struct MercurialInputScheme : InputScheme
 
         if (!input.getRef() && !input.getRev() && isLocal && pathExists(actualUrl + "/.hg")) {
 
-            bool clean = runProgram("hg", true, { "status", "-R", actualUrl, "--modified", "--added", "--removed" }) == "";
+            bool clean = runHg({ "status", "-R", actualUrl, "--modified", "--added", "--removed" }) == "";
 
             if (!clean) {
 
@@ -143,10 +173,10 @@ struct MercurialInputScheme : InputScheme
                 if (settings.warnDirty)
                     warn("Mercurial tree '%s' is unclean", actualUrl);
 
-                input.attrs.insert_or_assign("ref", chomp(runProgram("hg", true, { "branch", "-R", actualUrl })));
+                input.attrs.insert_or_assign("ref", chomp(runHg({ "branch", "-R", actualUrl })));
 
                 auto files = tokenizeString<std::set<std::string>>(
-                    runProgram("hg", true, { "status", "-R", actualUrl, "--clean", "--modified", "--added", "--no-status", "--print0" }), "\0"s);
+                    runHg({ "status", "-R", actualUrl, "--clean", "--modified", "--added", "--no-status", "--print0" }), "\0"s);
 
                 PathFilter filter = [&](const Path & p) -> bool {
                     assert(hasPrefix(p, actualUrl));
@@ -224,33 +254,33 @@ struct MercurialInputScheme : InputScheme
         if (!(input.getRev()
                 && pathExists(cacheDir)
                 && runProgram(
-                    RunOptions("hg", { "log", "-R", cacheDir, "-r", input.getRev()->gitRev(), "--template", "1" })
+                    hgOptions({ "log", "-R", cacheDir, "-r", input.getRev()->gitRev(), "--template", "1" })
                     .killStderr(true)).second == "1"))
         {
             Activity act(*logger, lvlTalkative, actUnknown, fmt("fetching Mercurial repository '%s'", actualUrl));
 
             if (pathExists(cacheDir)) {
                 try {
-                    runProgram("hg", true, { "pull", "-R", cacheDir, "--", actualUrl });
+                    runHg({ "pull", "-R", cacheDir, "--", actualUrl });
                 }
                 catch (ExecError & e) {
                     string transJournal = cacheDir + "/.hg/store/journal";
                     /* hg throws "abandoned transaction" error only if this file exists */
                     if (pathExists(transJournal)) {
-                        runProgram("hg", true, { "recover", "-R", cacheDir });
+                        runHg({ "recover", "-R", cacheDir });
-                        runProgram("hg", true, { "pull", "-R", cacheDir, "--", actualUrl });
+                        runHg({ "pull", "-R", cacheDir, "--", actualUrl });
                     } else {
                         throw ExecError(e.status, fmt("'hg pull' %s", statusToString(e.status)));
                     }
                 }
             } else {
                 createDirs(dirOf(cacheDir));
-                runProgram("hg", true, { "clone", "--noupdate", "--", actualUrl, cacheDir });
+                runHg({ "clone", "--noupdate", "--", actualUrl, cacheDir });
             }
         }
 
         auto tokens = tokenizeString<std::vector<std::string>>(
-            runProgram("hg", true, { "log", "-R", cacheDir, "-r", revOrRef, "--template", "{node} {rev} {branch}" }));
+            runHg({ "log", "-R", cacheDir, "-r", revOrRef, "--template", "{node} {rev} {branch}" }));
         assert(tokens.size() == 3);
 
         input.attrs.insert_or_assign("rev", Hash::parseAny(tokens[0], htSHA1).gitRev());
@@ -263,7 +293,7 @@ struct MercurialInputScheme : InputScheme
         Path tmpDir = createTempDir();
         AutoDelete delTmpDir(tmpDir, true);
 
-        runProgram("hg", true, { "archive", "-R", cacheDir, "-r", input.getRev()->gitRev(), tmpDir });
+        runHg({ "archive", "-R", cacheDir, "-r", input.getRev()->gitRev(), tmpDir });
 
         deletePath(tmpDir + "/.hg_archival.txt");
 

tests/fetchMercurial.sh

@@ -15,6 +15,9 @@ hg init $repo
 echo '[ui]' >> $repo/.hg/hgrc
 echo 'username = Foobar <foobar@example.org>' >> $repo/.hg/hgrc
 
+# Set ui.tweakdefaults to ensure HGPLAIN is being set.
+echo 'tweakdefaults = True' >> $repo/.hg/hgrc
+
 echo utrecht > $repo/hello
 touch $repo/.hgignore
 hg add --cwd $repo hello .hgignore