From 7112f8294c162db536b15f9d527033c9d641e057 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Thu, 23 May 2024 11:53:17 -0400
Subject: [PATCH 1/2] Add `SSHMaster::Connection::trySetBufferSize`

It is unused in Nix currently, but will be used in Hydra. This reflects
what Hydra does in https://github.com/NixOS/hydra/pull/1387.

We may probably to use it more widely for better SSH store performance,
but this needs to be subject to more testing before we do that.

(cherry picked from commit 0d25cc65417647c454e3095650b87bc88351b384)
---
 src/libstore/ssh.cc | 15 +++++++++++++++
 src/libstore/ssh.hh | 12 ++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/src/libstore/ssh.cc b/src/libstore/ssh.cc
index 116a480ba..f47cfbbec 100644
--- a/src/libstore/ssh.cc
+++ b/src/libstore/ssh.cc
@@ -240,4 +240,19 @@ Path SSHMaster::startMaster()
 
 #endif
 
+void SSHMaster::Connection::trySetBufferSize(size_t size)
+{
+#ifdef F_SETPIPE_SZ
+    /* This `fcntl` method of doing this takes a positive `int`. Check
+       and convert accordingly.
+
+       The function overall still takes `size_t` because this is more
+       portable for a platform-agnostic interface. */
+    assert(size <= INT_MAX);
+    int pipesize = size;
+    fcntl(in.get(), F_SETPIPE_SZ, pipesize);
+    fcntl(out.get(), F_SETPIPE_SZ, pipesize);
+#endif
+}
+
 }
diff --git a/src/libstore/ssh.hh b/src/libstore/ssh.hh
index 85be704ec..eb05df011 100644
--- a/src/libstore/ssh.hh
+++ b/src/libstore/ssh.hh
@@ -54,6 +54,18 @@ public:
         Pid sshPid;
 #endif
         AutoCloseFD out, in;
+
+        /**
+         * Try to set the buffer size in both directions to the
+         * designated amount, if possible. If not possible, does
+         * nothing.
+         *
+         * Current implementation is to use `fcntl` with `F_SETPIPE_SZ`,
+         * which is Linux-only. For this implementation, `size` must
+         * convertable to an `int`. In other words, it must be within
+         * `[0, INT_MAX]`.
+         */
+        void trySetBufferSize(size_t size);
     };
 
     /**

From fa7f0d6d07bdbedd06904d52bd111e58cb3d64c9 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Sun, 16 Feb 2025 20:01:03 -0500
Subject: [PATCH 2/2] Allow setting `ssh://` pipe size

Exposed for Hydra. We could make it fancier but with (a) new store
settings (b) switch to `ssh-ng://` both in the works, it doesn't seem
worth it.

(cherry picked from commit 94a7c34b2f8285650e3130e5dc6ff5333eaa6dc8)
---
 src/libstore/legacy-ssh-store.cc | 3 +++
 src/libstore/legacy-ssh-store.hh | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 3f62794ef..3849f088d 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -70,6 +70,9 @@ ref<LegacySSHStore::Connection> LegacySSHStore::openConnection()
         command.push_back(remoteStore.get());
     }
     conn->sshConn = master.startCommand(std::move(command), std::list{extraSshArgs});
+    if (connPipeSize) {
+        conn->sshConn->trySetBufferSize(*connPipeSize);
+    }
     conn->to = FdSink(conn->sshConn->in.get());
     conn->from = FdSource(conn->sshConn->out.get());
 
diff --git a/src/libstore/legacy-ssh-store.hh b/src/libstore/legacy-ssh-store.hh
index 2444a7a66..92aa4ae56 100644
--- a/src/libstore/legacy-ssh-store.hh
+++ b/src/libstore/legacy-ssh-store.hh
@@ -30,6 +30,11 @@ struct LegacySSHStoreConfig : virtual CommonSSHStoreConfig
      */
     Strings extraSshArgs = {};
 
+    /**
+     * Exposed for hydra
+     */
+    std::optional<size_t> connPipeSize;
+
     const std::string name() override { return "SSH Store"; }
 
     static std::set<std::string> uriSchemes() { return {"ssh"}; }