build-remote: Ugly hackery to get build logs to work

The build hook mechanism expects build log output to go to file
descriptor 4, so do that.

3 files changed, 10 insertions, 2 deletions

diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 1f6ea4dc1cfd..a6479a450e34 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -17,6 +17,9 @@ struct LegacySSHStore : public Store
     const Setting<Path> sshKey{this, "", "ssh-key", "path to an SSH private key"};
     const Setting<bool> compress{this, false, "compress", "whether to compress the connection"};
 
+    // Hack for getting remote build log output.
+    const Setting<int> logFD{this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"};
+
     struct Connection
     {
         std::unique_ptr<SSHMaster::Connection> sshConn;
@@ -44,7 +47,8 @@ struct LegacySSHStore : public Store
             sshKey,
             // Use SSH master only if using more than 1 connection.
             connections->capacity() > 1,
-            compress)
+            compress,
+            logFD)
     {
     }
 
diff --git a/src/libstore/ssh.cc b/src/libstore/ssh.cc
index e54f3f4ba284..6edabaa3a1d9 100644
--- a/src/libstore/ssh.cc
+++ b/src/libstore/ssh.cc
@@ -31,6 +31,8 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(const std::string
             throw SysError("duping over stdin");
         if (dup2(out.writeSide.get(), STDOUT_FILENO) == -1)
             throw SysError("duping over stdout");
+        if (logFD != -1 && dup2(logFD, STDERR_FILENO) == -1)
+            throw SysError("duping over stderr");
 
         Strings args = { "ssh", host.c_str(), "-x", "-a" };
         addCommonSSHOpts(args);
diff --git a/src/libstore/ssh.hh b/src/libstore/ssh.hh
index b4396467e54e..18dea227ad1f 100644
--- a/src/libstore/ssh.hh
+++ b/src/libstore/ssh.hh
@@ -13,6 +13,7 @@ private:
     const std::string keyFile;
     const bool useMaster;
     const bool compress;
+    const int logFD;
 
     struct State
     {
@@ -27,11 +28,12 @@ private:
 
 public:
 
-    SSHMaster(const std::string & host, const std::string & keyFile, bool useMaster, bool compress)
+    SSHMaster(const std::string & host, const std::string & keyFile, bool useMaster, bool compress, int logFD = -1)
         : host(host)
         , keyFile(keyFile)
         , useMaster(useMaster)
         , compress(compress)
+        , logFD(logFD)
     {
     }