From d74b54b3ddf710926a44bf3f9c87c00e6f82d825 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Fri, 5 Nov 2021 12:29:35 -0300
Subject: [PATCH] Avoid crash in rare case of concurrent DROP

When a role being dropped contains is referenced by catalog objects that
are concurrently also being dropped, a crash can result while trying to
construct the string that describes the objects.  Suppress that by
ignoring objects whose descriptions are returned as NULL.

The majority of relevant codesites were already cautious about this
already; we had just missed a couple.

This is an old bug, so backpatch all the way back.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/17126-21887f04508cb5c8@postgresql.org
---
 src/backend/catalog/dependency.c  | 31 ++++++++++++++++++++-----------
 src/backend/catalog/pg_shdepend.c |  6 ++++++
 2 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c
index 9f8eb1a37f..fe9c714257 100644
--- a/src/backend/catalog/dependency.c
+++ b/src/backend/catalog/dependency.c
@@ -1094,6 +1094,10 @@ reportDependentObjects(const ObjectAddresses *targetObjects,
 
 		objDesc = getObjectDescription(obj, false);
 
+		/* An object being dropped concurrently doesn't need to be reported */
+		if (objDesc == NULL)
+			continue;
+
 		/*
 		 * If, at any stage of the recursive search, we reached the object via
 		 * an AUTO, INTERNAL, PARTITION, or EXTENSION dependency, then it's
@@ -1119,23 +1123,28 @@ reportDependentObjects(const ObjectAddresses *targetObjects,
 			char	   *otherDesc = getObjectDescription(&extra->dependee,
 														 false);
 
-			if (numReportedClient < MAX_REPORTED_DEPS)
+			if (otherDesc)
 			{
+				if (numReportedClient < MAX_REPORTED_DEPS)
+				{
+					/* separate entries with a newline */
+					if (clientdetail.len != 0)
+						appendStringInfoChar(&clientdetail, '\n');
+					appendStringInfo(&clientdetail, _("%s depends on %s"),
+									 objDesc, otherDesc);
+					numReportedClient++;
+				}
+				else
+					numNotReportedClient++;
 				/* separate entries with a newline */
-				if (clientdetail.len != 0)
-					appendStringInfoChar(&clientdetail, '\n');
-				appendStringInfo(&clientdetail, _("%s depends on %s"),
+				if (logdetail.len != 0)
+					appendStringInfoChar(&logdetail, '\n');
+				appendStringInfo(&logdetail, _("%s depends on %s"),
 								 objDesc, otherDesc);
-				numReportedClient++;
+				pfree(otherDesc);
 			}
 			else
 				numNotReportedClient++;
-			/* separate entries with a newline */
-			if (logdetail.len != 0)
-				appendStringInfoChar(&logdetail, '\n');
-			appendStringInfo(&logdetail, _("%s depends on %s"),
-							 objDesc, otherDesc);
-			pfree(otherDesc);
 			ok = false;
 		}
 		else
diff --git a/src/backend/catalog/pg_shdepend.c b/src/backend/catalog/pg_shdepend.c
index 8453d8fefd..9ea42f805f 100644
--- a/src/backend/catalog/pg_shdepend.c
+++ b/src/backend/catalog/pg_shdepend.c
@@ -1234,6 +1234,12 @@ storeObjectDescription(StringInfo descs,
 {
 	char	   *objdesc = getObjectDescription(object, false);
 
+	/*
+	 * An object being dropped concurrently doesn't need to be reported.
+	 */
+	if (objdesc == NULL)
+		return;
+
 	/* separate entries with a newline */
 	if (descs->len != 0)
 		appendStringInfoChar(descs, '\n');