From 9b82793ddfa2d3f375eb64e37fb5b965b655b392 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 24 Jun 2026 11:20:01 +0100
Subject: [PATCH] Correctly handle libpq default host

---
 Cargo.lock                        | 246 +++++++++++++++++++++++++++---
 rust/Cargo.toml                   |   1 +
 rust/src/database/postgres/mod.rs |  31 +++-
 3 files changed, 251 insertions(+), 27 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index addf6f7300..1eceec8408 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -31,7 +31,7 @@ checksum = "9035ad2d096bed7955a320ee7e2230574d28fd3c3a0f186cbea1ff3c7eed5dbb"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -46,6 +46,34 @@ version = "0.22.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "72b3254f16251a8381aa12e40e3c4d2f0199f8c6508fbecb9d91f575e0fbb8c6"
 
+[[package]]
+name = "bindgen"
+version = "0.64.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c4243e6031260db77ede97ad86c27e501d646a27ab57b59a574f725d98ab1fb4"
+dependencies = [
+ "bitflags 1.3.2",
+ "cexpr",
+ "clang-sys",
+ "lazy_static",
+ "lazycell",
+ "log",
+ "peeking_take_while",
+ "proc-macro2",
+ "quote",
+ "regex",
+ "rustc-hash 1.1.0",
+ "shlex",
+ "syn 1.0.109",
+ "which",
+]
+
+[[package]]
+name = "bitflags"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
+
 [[package]]
 name = "bitflags"
 version = "2.9.1"
@@ -106,6 +134,15 @@ dependencies = [
  "shlex",
 ]
 
+[[package]]
+name = "cexpr"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6fac387a98bb7c37292057cffc56d62ecb629900026402633ae9160df93a8766"
+dependencies = [
+ "nom",
+]
+
 [[package]]
 name = "cfg-if"
 version = "1.0.1"
@@ -129,6 +166,17 @@ dependencies = [
  "rand_core 0.10.1",
 ]
 
+[[package]]
+name = "clang-sys"
+version = "1.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b023947811758c97c59bf9d1c188fd619ad4718dcaa767947df1cadb14f39f4"
+dependencies = [
+ "glob",
+ "libc",
+ "libloading",
+]
+
 [[package]]
 name = "cmov"
 version = "0.5.4"
@@ -243,7 +291,7 @@ checksum = "97369cbbc041bc366949bc74d34658d6cda5621039731c6310521892a3a20ae0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -258,6 +306,16 @@ version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "877a4ace8713b0bcf2a4e7eec82529c029f1d0619886d18145fea96c3ffe5c0f"
 
+[[package]]
+name = "errno"
+version = "0.3.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
+dependencies = [
+ "libc",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "fallible-iterator"
 version = "0.2.0"
@@ -341,7 +399,7 @@ checksum = "e835b70203e41293343137df5c0664546da5745f82ec9b84d40be8336958447b"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -424,6 +482,12 @@ dependencies = [
  "wasip3",
 ]
 
+[[package]]
+name = "glob"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0cc23270f6e1808e30a928bdc84dea0b9b4136a8bc82338574f23baf47bbd280"
+
 [[package]]
 name = "h2"
 version = "0.4.11"
@@ -497,6 +561,15 @@ dependencies = [
  "digest 0.11.3",
 ]
 
+[[package]]
+name = "home"
+version = "0.5.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cc627f471c528ff0c4a49e1d5e60450c8f6461dd6d10ba9dcd3a61d3dff7728d"
+dependencies = [
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "http"
 version = "1.4.0"
@@ -831,6 +904,12 @@ version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
 
+[[package]]
+name = "lazycell"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "830d08ce1d1d941e6b30645f1a0eb5643013d835ce3779a5fc208261dbe10f55"
+
 [[package]]
 name = "leb128fmt"
 version = "0.1.0"
@@ -843,12 +922,46 @@ version = "0.2.186"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "68ab91017fe16c622486840e4c83c9a37afeff978bd239b5293d61ece587de66"
 
+[[package]]
+name = "libloading"
+version = "0.8.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d7c4b02199fee7c5d21a5ae7d8cfa79a6ef5bb2fc834d6e9058e89c825efdc55"
+dependencies = [
+ "cfg-if",
+ "windows-link",
+]
+
 [[package]]
 name = "libm"
 version = "0.2.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f9fbbcab51052fe104eb5e5d351cf728d30a5be1fe14d9be8a3b097481fb97de"
 
+[[package]]
+name = "libpq"
+version = "6.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e4d79c9e41ea45340f5f851479405db923d553a5885aaf8d3c81a4ace8fcb5ad"
+dependencies = [
+ "bitflags 2.9.1",
+ "libc",
+ "libpq-sys",
+ "log",
+ "thiserror",
+]
+
+[[package]]
+name = "libpq-sys"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2ef060ac05c207c85da15f4eb629100c8782e0db4c06a3c91c86be9c18ae8a23"
+dependencies = [
+ "bindgen",
+ "pkg-config",
+ "vcpkg",
+]
+
 [[package]]
 name = "libredox"
 version = "0.1.16"
@@ -858,6 +971,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "linux-raw-sys"
+version = "0.4.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d26c52dbd32dccf2d10cac7725f8eae5296885fb5703b261f7d0a0739ec807ab"
+
 [[package]]
 name = "litemap"
 version = "0.8.0"
@@ -907,6 +1026,12 @@ version = "0.3.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6877bb514081ee2a7ff5ef9de3281f14a4dd4bceac4c09388074a6b5df8a139a"
 
+[[package]]
+name = "minimal-lexical"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "68354c5c6bd36d73ff3feceb05efa59b6acb7626617f4962be322a825e61f79a"
+
 [[package]]
 name = "mio"
 version = "1.0.4"
@@ -918,13 +1043,23 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "nom"
+version = "7.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d273983c5a657a70a3e8f2a01329822f3b8c8172b73826411a55751e404a0a4a"
+dependencies = [
+ "memchr",
+ "minimal-lexical",
+]
+
 [[package]]
 name = "objc2-core-foundation"
 version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2a180dd8642fa45cdb7dd721cd4c11b1cadd4929ce112ebd8b9f5803cc79d536"
 dependencies = [
- "bitflags",
+ "bitflags 2.9.1",
 ]
 
 [[package]]
@@ -971,6 +1106,12 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "peeking_take_while"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "19b17cddbe7ec3f8bc800887bab5e717348c95ea2ca0b1bf0837fb964dc67099"
+
 [[package]]
 name = "percent-encoding"
 version = "2.3.1"
@@ -1002,6 +1143,12 @@ version = "0.2.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3b3cff922bd51709b605d9ead9aa71031d81447142d828eb4a6eba76fe619f9b"
 
+[[package]]
+name = "pkg-config"
+version = "0.3.33"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "19f132c84eca552bf34cab8ec81f1c1dcc229b811638f9d283dceabe58c5569e"
+
 [[package]]
 name = "portable-atomic"
 version = "1.11.1"
@@ -1063,7 +1210,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "479ca8adacdd7ce8f1fb39ce9ecccbfe93a3f1344b3d0d97f20bc0196208f62b"
 dependencies = [
  "proc-macro2",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -1130,7 +1277,7 @@ dependencies = [
  "proc-macro2",
  "pyo3-macros-backend",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -1143,7 +1290,7 @@ dependencies = [
  "proc-macro2",
  "pyo3-build-config",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -1168,7 +1315,7 @@ dependencies = [
  "pin-project-lite",
  "quinn-proto",
  "quinn-udp",
- "rustc-hash",
+ "rustc-hash 2.1.1",
  "rustls",
  "socket2 0.5.10",
  "thiserror",
@@ -1188,7 +1335,7 @@ dependencies = [
  "lru-slab",
  "rand 0.9.4",
  "ring",
- "rustc-hash",
+ "rustc-hash 2.1.1",
  "rustls",
  "rustls-pki-types",
  "slab",
@@ -1285,7 +1432,7 @@ version = "0.5.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ed2bf2547551a7053d6fdfafda3f938979645c44812fbfcda098faae3f1a362d"
 dependencies = [
- "bitflags",
+ "bitflags 2.9.1",
 ]
 
 [[package]]
@@ -1373,6 +1520,12 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "rustc-hash"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "08d43f7aa6b08d49f382cde6a7982047c3426db949b1424bc4b7ec9ae12c6ce2"
+
 [[package]]
 name = "rustc-hash"
 version = "2.1.1"
@@ -1388,6 +1541,19 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "rustix"
+version = "0.38.44"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fdb5bc1ae2baa591800df16c9ca78619bf65c0488b41b96ccec5d11220d8c154"
+dependencies = [
+ "bitflags 2.9.1",
+ "errno",
+ "libc",
+ "linux-raw-sys",
+ "windows-sys 0.59.0",
+]
+
 [[package]]
 name = "rustls"
 version = "0.23.31"
@@ -1468,7 +1634,7 @@ version = "3.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "271720403f46ca04f7ba6f55d438f8bd878d6b8ca0a1046e8228c4145bcbb316"
 dependencies = [
- "bitflags",
+ "bitflags 2.9.1",
  "core-foundation",
  "core-foundation-sys",
  "libc",
@@ -1518,7 +1684,7 @@ checksum = "d540f220d3187173da220f885ab66608367b6574e925011a9353e4badda91d79"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -1646,6 +1812,17 @@ version = "2.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "13c2bddecc57b384dee18652358fb23172facb8a2c51ccc10d74c157bdea3292"
 
+[[package]]
+name = "syn"
+version = "1.0.109"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b64191b275b66ffe2469e8af2c1cfe3bafa67b529ead792a6d0160888b4237"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
 [[package]]
 name = "syn"
 version = "2.0.117"
@@ -1673,6 +1850,7 @@ dependencies = [
  "icu_segmenter",
  "itertools",
  "lazy_static",
+ "libpq",
  "log",
  "mime",
  "once_cell",
@@ -1708,7 +1886,7 @@ checksum = "728a70f3dbaf5bab7f0c4b1ac8d7ae5ea60a4b5549c8a5914361c99147a709d2"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -1734,7 +1912,7 @@ checksum = "7f7cf42b4507d8ea322120659672cf1b9dbb93f8f2d4ecfd6e51350ff5b17a1d"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -1846,7 +2024,7 @@ version = "0.6.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d4e6559d53cc268e5031cd8429d05415bc4cb4aefc4aa5d6cc35fbf5b924a1f8"
 dependencies = [
- "bitflags",
+ "bitflags 2.9.1",
  "bytes",
  "futures-util",
  "http",
@@ -1967,6 +2145,12 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b6c140620e7ffbb22c2dee59cafe6084a59b5ffc27a8859a5f0d494b5d52b6be"
 
+[[package]]
+name = "vcpkg"
+version = "0.2.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "accd4ea62f7bb7a82fe23066fb0957d48ef677f6eeb8215f372f52e48bb32426"
+
 [[package]]
 name = "version_check"
 version = "0.9.5"
@@ -2066,7 +2250,7 @@ dependencies = [
  "bumpalo",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
  "wasm-bindgen-shared",
 ]
 
@@ -2120,7 +2304,7 @@ version = "0.244.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "47b807c72e1bac69382b3a6fb3dbe8ea4c0ed87ff5629b8685ae6b9a611028fe"
 dependencies = [
- "bitflags",
+ "bitflags 2.9.1",
  "hashbrown",
  "indexmap",
  "semver",
@@ -2146,6 +2330,18 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "which"
+version = "4.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "87ba24419a2078cd2b0f2ede2691b6c66d8e47836da3b6db8265ebad47afbfc7"
+dependencies = [
+ "either",
+ "home",
+ "once_cell",
+ "rustix",
+]
+
 [[package]]
 name = "whoami"
 version = "2.1.2"
@@ -2288,7 +2484,7 @@ version = "0.39.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6f42320e61fe2cfd34354ecb597f86f413484a798ba44a8ca1165c58d42da6c1"
 dependencies = [
- "bitflags",
+ "bitflags 2.9.1",
 ]
 
 [[package]]
@@ -2301,7 +2497,7 @@ dependencies = [
  "heck",
  "indexmap",
  "prettyplease",
- "syn",
+ "syn 2.0.117",
  "wasm-metadata",
  "wit-bindgen-core",
  "wit-component",
@@ -2317,7 +2513,7 @@ dependencies = [
  "prettyplease",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
  "wit-bindgen-core",
  "wit-bindgen-rust",
 ]
@@ -2329,7 +2525,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9d66ea20e9553b30172b5e831994e35fbde2d165325bec84fc43dbf6f4eb9cb2"
 dependencies = [
  "anyhow",
- "bitflags",
+ "bitflags 2.9.1",
  "indexmap",
  "log",
  "serde",
@@ -2385,7 +2581,7 @@ checksum = "38da3c9736e16c5d3c8c597a9aaa5d1fa565d0532ae05e27c24aa62fb32c0ab6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
  "synstructure",
 ]
 
@@ -2406,7 +2602,7 @@ checksum = "9ecf5b4cc5364572d7f4c329661bcc82724222973f2cab6f050a4e5c22f75181"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
@@ -2426,7 +2622,7 @@ checksum = "d71e5d6e06ab090c67b5e44993ec16b72dcbaabc526db883a360057678b48502"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
  "synstructure",
 ]
 
@@ -2466,7 +2662,7 @@ checksum = "5b96237efa0c878c64bd89c436f661be4e46b2f3eff1ebb976f7ef2321d2f58f"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.117",
 ]
 
 [[package]]
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 4f3d00ee70..d9cf08e121 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -65,6 +65,7 @@ tokio-postgres = "0.7"
 once_cell = "1.18.0"
 itertools = "0.14.0"
 postgres-protocol = "0.6.10"
+libpq = "6.0.1"
 
 [build-dependencies]
 blake2 = "0.10.4"
diff --git a/rust/src/database/postgres/mod.rs b/rust/src/database/postgres/mod.rs
index 5be089b990..4865744f42 100644
--- a/rust/src/database/postgres/mod.rs
+++ b/rust/src/database/postgres/mod.rs
@@ -5,6 +5,7 @@
 //! methods are kept `pub` so that future Rust callers can drive them
 //! directly without going through the PyO3 wrappers.
 
+use anyhow::Error;
 use log::warn;
 use pyo3::exceptions::PyRuntimeError;
 use pyo3::prelude::*;
@@ -42,8 +43,10 @@ fn pg_err_to_py(e: tokio_postgres::Error) -> PyErr {
 
 #[pyfunction]
 fn connect<'py>(py: Python<'py>, dsn: &str) -> PyResult<Bound<'py, connection::Connection>> {
-    let (client, connection) =
-        tokio_postgres::connect(dsn, tokio_postgres::NoTls).block_on_result(py)?;
+    let config = fixup_default_host(dsn)
+        .map_err(|e| PyRuntimeError::new_err(format!("Failed to parse DSN: {e}")))?;
+
+    let (client, connection) = config.connect(tokio_postgres::NoTls).block_on_result(py)?;
 
     // Spawn the connection task on the runtime.
     runtime().spawn(async move {
@@ -56,3 +59,27 @@ fn connect<'py>(py: Python<'py>, dsn: &str) -> PyResult<Bound<'py, connection::C
 
     Bound::new(py, conn)
 }
+
+/// Fix up a DSN to ensure it has a host, using libpq's default host if
+/// necessary.
+///
+/// [`tokio-postgres`] has a different default host than [`libpq`], which is
+/// what Synapse previously used (and is what e.g. `psql` uses). The default
+/// host in `libpq` is configurable, and so we need to pull it out and runtime.
+fn fixup_default_host(dsn: &str) -> Result<tokio_postgres::Config, Error> {
+    let mut config = dsn.parse::<tokio_postgres::Config>()?;
+
+    if !config.get_hosts().is_empty() {
+        return Ok(config);
+    }
+
+    // Get the default host from libpq. This will *not* do any network I/O, it
+    // just parses the DSN. The connection will get closed automatically on
+    // Drop.
+    let pq_conn = libpq::Connection::new("")?;
+    let host = pq_conn.host()?;
+
+    config.host(host);
+
+    Ok(config)
+}