Hi Hackers,
I'm submitting a patch to fix a bug where ALTER SYSTEM SET with empty
strings for
GUC_LIST_QUOTE parameters (like shared_preload_libraries) results in
malformed
configuration entries that cause server crashes on restart.
Please take a look,
Thanks
Andrew
# Fix for ALTER SYSTEM Empty String Bug
## Problem Description
When using `ALTER SYSTEM SET "shared_preload_libraries" TO ''`, PostgreSQL would write a malformed configuration entry to `postgresql.auto.conf`:
```
shared_preload_libraries = '""'
```
This malformed configuration would cause the PostgreSQL server to crash on restart.
## Root Cause
The issue occurred in the `flatten_set_variable_args` function in `src/backend/utils/misc/guc_funcs.c`. When a parameter with the `GUC_LIST_QUOTE` flag (like `shared_preload_libraries`) was set to an empty string, the function would call `quote_identifier("")` which returns `""` (double quotes around an empty string). This resulted in the malformed configuration entry.
## Solution
The fix modifies the `flatten_set_variable_args` function to check if the value is an empty string before applying the `GUC_LIST_QUOTE` logic. Empty strings are treated as "no value" rather than literal empty strings, so they are not quoted.
## Code Changes
In `src/backend/utils/misc/guc_funcs.c`, line 291:
```c
- if (flags & GUC_LIST_QUOTE)
+ if ((flags & GUC_LIST_QUOTE) && val[0] != '\0')
```
## Testing
The fix can be tested by:
1. Running `ALTER SYSTEM SET "shared_preload_libraries" TO ''`
2. Checking that `postgresql.auto.conf` does not contain malformed entries
3. Restarting PostgreSQL to verify no crashes occur
## Affected Parameters
This fix affects all GUC parameters that have the `GUC_LIST_QUOTE` flag set, including:
- `shared_preload_libraries`
- Any other list-type parameters that use quote mode
From 3a6a7db602036309fbc6a7f5c7c731a71038967b Mon Sep 17 00:00:00 2001
From: Andrew Klychkov <andrew.a.klych...@gmail.com>
Date: Thu, 28 Aug 2025 10:33:42 +0200
Subject: [PATCH] Fix ALTER SYSTEM empty string bug for GUC_LIST_QUOTE
parameters
When ALTER SYSTEM SET is used with an empty string for parameters with
GUC_LIST_QUOTE flag (like shared_preload_libraries), the empty string
was being quoted by quote_identifier(), resulting in '' being written
to postgresql.auto.conf. This caused server crashes on restart.
The fix prevents empty strings from being quoted when GUC_LIST_QUOTE is
set, treating them as 'no value' rather than literal empty strings.
Fixes bug where 'ALTER SYSTEM SET "shared_preload_libraries" TO '''
would write 'shared_preload_libraries = '""'' to postgresql.auto.conf.
---
src/backend/utils/misc/guc_funcs.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/src/backend/utils/misc/guc_funcs.c b/src/backend/utils/misc/guc_funcs.c
index b9e26982ab..389841a4ec 100644
--- a/src/backend/utils/misc/guc_funcs.c
+++ b/src/backend/utils/misc/guc_funcs.c
@@ -288,8 +288,10 @@ flatten_set_variable_args(const char *name, List *args)
/*
* Plain string literal or identifier. For quote mode,
* quote it if it's not a vanilla identifier.
+ * However, empty strings should not be quoted as they
+ * represent "no value" rather than a literal empty string.
*/
- if (flags & GUC_LIST_QUOTE)
+ if ((flags & GUC_LIST_QUOTE) && val[0] != '\0')
appendStringInfoString(&buf, quote_identifier(val));
else
appendStringInfoString(&buf, val);
--
2.47.0
