Support netrc in <nix/fetchurl.nix>

This allows <nix/fetchurl.nix> to fetch private Git/Mercurial
repositories, e.g.

  import <nix/fetchurl.nix> {
    url = https://edolstra@bitbucket.org/edolstra/my-private-repo/get/80a14018daed.tar.bz2;
    sha256 = "1mgqzn7biqkq3hf2697b0jc4wabkqhmzq2srdymjfa6sb9zb6qs7";
  }

where /etc/nix/netrc contains:

  machine bitbucket.org
  login edolstra
  password blabla...

This works even when sandboxing is enabled.

To do: add unpacking support (i.e. fetchzip functionality).

src/libstore/build.cc

@@ -2307,6 +2307,14 @@ void DerivationGoal::runChild()
 
         bool setUser = true;
 
+        /* Make the contents of netrc available to builtin:fetchurl
+           (which may run under a different uid and/or in a sandbox). */
+        std::string netrcData;
+        try {
+            if (drv->isBuiltin() && drv->builder == "builtin:fetchurl")
+                netrcData = readFile(settings.netrcFile);
+        } catch (SysError &) { }
+
 #if __linux__
         if (useChroot) {
 
@@ -2675,7 +2683,7 @@ void DerivationGoal::runChild()
         if (drv->isBuiltin()) {
             try {
                 if (drv->builder == "builtin:fetchurl")
-                    builtinFetchurl(*drv);
+                    builtinFetchurl(*drv, netrcData);
                 else
                     throw Error(format("unsupported builtin function ‘%1%’") % string(drv->builder, 8));
                 _exit(0);
@@ -3072,7 +3080,9 @@ void DerivationGoal::closeLogFile()
 void DerivationGoal::deleteTmpDir(bool force)
 {
     if (tmpDir != "") {
-        if (settings.keepFailed && !force) {
+        /* Don't keep temporary directories for builtins because they
+           might have privileged stuff (like a copy of netrc). */
+        if (settings.keepFailed && !force && !drv->isBuiltin()) {
             printError(
                 format("note: keeping build directory ‘%2%’")
                 % drvPath % tmpDir);

src/libstore/builtins.cc

@@ -6,8 +6,16 @@
 
 namespace nix {
 
-void builtinFetchurl(const BasicDerivation & drv)
+void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
 {
+    /* Make the host's netrc data available. Too bad curl requires
+       this to be stored in a file. It would be nice if we could just
+       pass a pointer to the data. */
+    if (netrcData != "") {
+        settings.netrcFile = "netrc";
+        writeFile(settings.netrcFile, netrcData, 0600);
+    }
+
     auto getAttr = [&](const string & name) {
         auto i = drv.env.find(name);
         if (i == drv.env.end()) throw Error(format("attribute ‘%s’ missing") % name);

src/libstore/builtins.hh

@@ -4,6 +4,6 @@
 
 namespace nix {
 
-void builtinFetchurl(const BasicDerivation & drv);
+void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData);
 
 }

src/libutil/util.cc

@@ -288,9 +288,9 @@ string readFile(const Path & path, bool drain)
 }
 
 
-void writeFile(const Path & path, const string & s)
+void writeFile(const Path & path, const string & s, mode_t mode)
 {
-    AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, 0666);
+    AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
         throw SysError(format("opening file ‘%1%’") % path);
     writeFull(fd.get(), s);

src/libutil/util.hh

@@ -89,7 +89,7 @@ string readFile(int fd);
 string readFile(const Path & path, bool drain = false);
 
 /* Write a string to a file. */
-void writeFile(const Path & path, const string & s);
+void writeFile(const Path & path, const string & s, mode_t mode = 0666);
 
 /* Read a line from a file descriptor. */
 string readLine(int fd);