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

@@ -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

pkgs/servers/sql/postgresql/pg_config.sh

@@ -0,0 +1,35 @@
+# 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 redirecting the call to pg_config to the one found in PATH,
+# iff we can be convinced that it belongs to our -dev output.
+
+# Avoid infinite recursion
+if [[ ! -v PG_CONFIG_CALLED ]]; then
+    # compares "path of *this* script" with "path, which pg_config on PATH believes it is in"
+    if [[ "$(readlink -f -- "$0")" == "$(PG_CONFIG_CALLED=1 pg_config --bindir)/pg_config" ]]; then
+        # The pg_config in PATH returns the same bindir that we're actually called from.
+        # This means that the pg_config in PATH is the one from "our" -dev output.
+        # 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.
+        exec pg_config "$@"
+    fi
+fi
+
+# This will happen in one of these cases:
+# - *this* script is the first on PATH
+# - np pg_config on PATH
+# - some other pg_config on PATH, not from our -dev output
+echo The real pg_config can be found in the -dev output.
+exit 1