From 9761c719c8a8c442285bea98bd60d406dd2df109 Mon Sep 17 00:00:00 2001
From: Wolfgang Walther <walther@technowledgy.de>
Date: Thu, 12 Sep 2024 19:15:29 +0200
Subject: [PATCH] postgresql: improve fake pg_config in default output

This fixes some build systems which look up the location of pg_config
via the location of the postgres binary itself, e.g. timescaledb,
instead of calling pg_config which is on PATH.

Since the -dev output is correctly placed before the default output of
postgresql in PATH, we can rely on that and call "pg_config" from the
default output's fake script. Only do that, when the one on PATH is
actually a different file, though, to prevent infinite loops.

Resolves #341408
---
 pkgs/servers/sql/postgresql/generic.nix  | 19 ++++++++-------
 pkgs/servers/sql/postgresql/pg_config.sh | 30 ++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 9 deletions(-)
 create mode 100644 pkgs/servers/sql/postgresql/pg_config.sh

diff --git a/pkgs/servers/sql/postgresql/generic.nix b/pkgs/servers/sql/postgresql/generic.nix
index de9dc49a09e1c09..ce922e807667d41 100644
--- a/pkgs/servers/sql/postgresql/generic.nix
+++ b/pkgs/servers/sql/postgresql/generic.nix
@@ -6,7 +6,8 @@ let
       , glibc, zlib, readline, openssl, icu, lz4, zstd, systemdLibs, libossp_uuid
       , pkg-config, libxml2, tzdata, libkrb5, substituteAll, darwin
       , linux-pam
-      , removeReferencesTo
+
+      , removeReferencesTo, writeShellApplication
 
       # This is important to obtain a version of `libpq` that does not depend on systemd.
       , systemdSupport ? lib.meta.availableOn stdenv.hostPlatform systemdLibs && !stdenv.hostPlatform.isStatic
@@ -54,6 +55,11 @@ let
         })
       else
         stdenv;
+
+    pg_config = writeShellApplication {
+      name = "pg_config";
+      text = builtins.readFile ./pg_config.sh;
+    };
   in stdenv'.mkDerivation (finalAttrs: {
     inherit version;
     pname = pname + lib.optionalString jitSupport "-jit";
@@ -201,15 +207,10 @@ let
         moveToOutput "lib/pgxs" "$dev"
 
         # Pretend pg_config is located in $out/bin to return correct paths, but
-        # actually have it in -dev to avoid pulling in all other outputs.
+        # actually have it in -dev to avoid pulling in all other outputs. See the
+        # pg_config.sh script's comments for details.
         moveToOutput "bin/pg_config" "$dev"
-        # To prevent a "pg_config: could not find own program executable" error, we fake
-        # pg_config in the default output.
-        cat << EOF > "$out/bin/pg_config" && chmod +x "$out/bin/pg_config"
-        #!${stdenv'.shell}
-        echo The real pg_config can be found in the -dev output.
-        exit 1
-        EOF
+        install -c -m 755 "${pg_config}"/bin/pg_config "$out/bin/pg_config"
         wrapProgram "$dev/bin/pg_config" --argv0 "$out/bin/pg_config"
 
         # postgres exposes external symbols get_pkginclude_path and similar. Those
diff --git a/pkgs/servers/sql/postgresql/pg_config.sh b/pkgs/servers/sql/postgresql/pg_config.sh
new file mode 100644
index 000000000000000..60da284f5ec601c
--- /dev/null
+++ b/pkgs/servers/sql/postgresql/pg_config.sh
@@ -0,0 +1,30 @@
+# The real pg_config needs to be in the same path as the "postgres" binary
+# to return proper paths. However, we want it in the -dev output to prevent
+# cyclic references and to prevent blowing up the runtime closure. Thus, we
+# have wrapped -dev/bin/pg_config to fake its argv0 to be in the default
+# output. Unfortunately, pg_config tries to be smart and tries to find itself -
+# which will then fail with:
+#   pg_config: could not find own program executable
+# To counter this, we're creating *this* fake pg_config script and put it into
+# the default output. The real pg_config is happy.
+# Some extensions, e.g. timescaledb, use the reverse logic and look for pg_config
+# in the same path as the "postgres" binary to support multi-version-installs.
+# Thus, they will end up calling this script during build, even though the real
+# pg_config would be available on PATH, provided by nativeBuildInputs. To help
+# this case, we're checking whether *this* script is the same as the one available
+# in PATH, and if not we're redirecting the call there.
+# If this script is called directly, we can't do anything except to error out.
+
+# compares "path of *this* script" with "path of pg_config on PATH"
+if [[ "$(readlink -f -- "$0")" != "$(readlink -f -- "$(type -P pg_config)")" ]]; then
+  # There is a different pg_config in PATH, let's call this instead.
+  # This happens when the -dev output has been put in native build
+  # inputs and allows us to call the real pg_config without referencing
+  # the -dev output itself.
+  pg_config "$@"
+else
+  # This will happen if either *this* script is the first on PATH or if there was
+  # no pg_config on PATH at all.
+  echo The real pg_config can be found in the -dev output.
+  exit 1
+fi