Prevent write operations on large objects in read-only transactions

Attempting such an operation would already fail, but in various and
confusing ways.  For example, while in recovery, some elog() messages
would be reported, but these should never be user-facing.  This commit
restricts any write operations done on large objects in a read-only
context, so as the errors generated are more user-friendly.  This is per
the discussion done with Tom Lane and Robert Haas.

Some regression tests are added to check the case of all the SQL
functions working on large objects (including an update of the test's
alternate output).

Author: Yugo Nagata
Discussion: https://postgr.es/m/20220527153028.61a4608f66abcd026fd3806f@sraoss.co.jp

doc/src/sgml/lobj.sgml

@@ -114,7 +114,9 @@
     All large object manipulation using these functions
     <emphasis>must</emphasis> take place within an SQL transaction block,
     since large object file descriptors are only valid for the duration of
-    a transaction.
+    a transaction. Write operations, including <function>lo_open</function>
+    with the <symbol>INV_WRITE</symbol> mode, are not allowed in a read-only
+    transaction.
    </para>
 
    <para>

src/backend/libpq/be-fsstubs.c

@@ -93,6 +93,9 @@ be_lo_open(PG_FUNCTION_ARGS)
 	elog(DEBUG4, "lo_open(%u,%d)", lobjId, mode);
 #endif
 
+	if (mode & INV_WRITE)
+		PreventCommandIfReadOnly("lo_open(INV_WRITE)");
+
 	/*
 	 * Allocate a large object descriptor first.  This will also create
 	 * 'fscxt' if this is the first LO opened in this transaction.
@@ -245,6 +248,8 @@ be_lo_creat(PG_FUNCTION_ARGS)
 {
 	Oid			lobjId;
 
+	PreventCommandIfReadOnly("lo_creat()");
+
 	lo_cleanup_needed = true;
 	lobjId = inv_create(InvalidOid);
 
@@ -256,6 +261,8 @@ be_lo_create(PG_FUNCTION_ARGS)
 {
 	Oid			lobjId = PG_GETARG_OID(0);
 
+	PreventCommandIfReadOnly("lo_create()");
+
 	lo_cleanup_needed = true;
 	lobjId = inv_create(lobjId);
 
@@ -306,6 +313,8 @@ be_lo_unlink(PG_FUNCTION_ARGS)
 {
 	Oid			lobjId = PG_GETARG_OID(0);
 
+	PreventCommandIfReadOnly("lo_unlink()");
+
 	/*
 	 * Must be owner of the large object.  It would be cleaner to check this
 	 * in inv_drop(), but we want to throw the error before not after closing
@@ -368,6 +377,8 @@ be_lowrite(PG_FUNCTION_ARGS)
 	int			bytestowrite;
 	int			totalwritten;
 
+	PreventCommandIfReadOnly("lowrite()");
+
 	bytestowrite = VARSIZE_ANY_EXHDR(wbuf);
 	totalwritten = lo_write(fd, VARDATA_ANY(wbuf), bytestowrite);
 	PG_RETURN_INT32(totalwritten);
@@ -413,6 +424,8 @@ lo_import_internal(text *filename, Oid lobjOid)
 	LargeObjectDesc *lobj;
 	Oid			oid;
 
+	PreventCommandIfReadOnly("lo_import()");
+
 	/*
 	 * open the file to be read in
 	 */
@@ -561,6 +574,8 @@ be_lo_truncate(PG_FUNCTION_ARGS)
 	int32		fd = PG_GETARG_INT32(0);
 	int32		len = PG_GETARG_INT32(1);
 
+	PreventCommandIfReadOnly("lo_truncate()");
+
 	lo_truncate_internal(fd, len);
 	PG_RETURN_INT32(0);
 }
@@ -571,6 +586,8 @@ be_lo_truncate64(PG_FUNCTION_ARGS)
 	int32		fd = PG_GETARG_INT32(0);
 	int64		len = PG_GETARG_INT64(1);
 
+	PreventCommandIfReadOnly("lo_truncate64()");
+
 	lo_truncate_internal(fd, len);
 	PG_RETURN_INT32(0);
 }
@@ -815,6 +832,8 @@ be_lo_from_bytea(PG_FUNCTION_ARGS)
 	LargeObjectDesc *loDesc;
 	int			written PG_USED_FOR_ASSERTS_ONLY;
 
+	PreventCommandIfReadOnly("lo_from_bytea()");
+
 	lo_cleanup_needed = true;
 	loOid = inv_create(loOid);
 	loDesc = inv_open(loOid, INV_WRITE, CurrentMemoryContext);
@@ -837,6 +856,8 @@ be_lo_put(PG_FUNCTION_ARGS)
 	LargeObjectDesc *loDesc;
 	int			written PG_USED_FOR_ASSERTS_ONLY;
 
+	PreventCommandIfReadOnly("lo_put()");
+
 	lo_cleanup_needed = true;
 	loDesc = inv_open(loOid, INV_WRITE, CurrentMemoryContext);
 

src/test/regress/expected/largeobject.out

@@ -506,6 +506,55 @@ SELECT lo_create(2121);
 (1 row)
 
 COMMENT ON LARGE OBJECT 2121 IS 'testing comments';
+-- Test writes on large objects in read-only transactions
+START TRANSACTION READ ONLY;
+-- INV_READ ... ok
+SELECT lo_open(2121, x'40000'::int);
+ lo_open 
+---------
+       0
+(1 row)
+
+-- INV_WRITE ... error
+SELECT lo_open(2121, x'20000'::int);
+ERROR:  cannot execute lo_open(INV_WRITE) in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_create(42);
+ERROR:  cannot execute lo_create() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_creat(42);
+ERROR:  cannot execute lo_creat() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_unlink(42);
+ERROR:  cannot execute lo_unlink() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lowrite(42, 'x');
+ERROR:  cannot execute lowrite() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_import(:'filename');
+ERROR:  cannot execute lo_import() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_truncate(42, 0);
+ERROR:  cannot execute lo_truncate() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_truncate64(42, 0);
+ERROR:  cannot execute lo_truncate64() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_from_bytea(0, 'x');
+ERROR:  cannot execute lo_from_bytea() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_put(42, 0, 'x');
+ERROR:  cannot execute lo_put() in a read-only transaction
+ROLLBACK;
 -- Clean up
 DROP TABLE lotest_stash_values;
 DROP ROLE regress_lo_user;

src/test/regress/expected/largeobject_1.out

@@ -506,6 +506,55 @@ SELECT lo_create(2121);
 (1 row)
 
 COMMENT ON LARGE OBJECT 2121 IS 'testing comments';
+-- Test writes on large objects in read-only transactions
+START TRANSACTION READ ONLY;
+-- INV_READ ... ok
+SELECT lo_open(2121, x'40000'::int);
+ lo_open 
+---------
+       0
+(1 row)
+
+-- INV_WRITE ... error
+SELECT lo_open(2121, x'20000'::int);
+ERROR:  cannot execute lo_open(INV_WRITE) in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_create(42);
+ERROR:  cannot execute lo_create() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_creat(42);
+ERROR:  cannot execute lo_creat() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_unlink(42);
+ERROR:  cannot execute lo_unlink() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lowrite(42, 'x');
+ERROR:  cannot execute lowrite() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_import(:'filename');
+ERROR:  cannot execute lo_import() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_truncate(42, 0);
+ERROR:  cannot execute lo_truncate() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_truncate64(42, 0);
+ERROR:  cannot execute lo_truncate64() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_from_bytea(0, 'x');
+ERROR:  cannot execute lo_from_bytea() in a read-only transaction
+ROLLBACK;
+START TRANSACTION READ ONLY;
+SELECT lo_put(42, 0, 'x');
+ERROR:  cannot execute lo_put() in a read-only transaction
+ROLLBACK;
 -- Clean up
 DROP TABLE lotest_stash_values;
 DROP ROLE regress_lo_user;

src/test/regress/sql/largeobject.sql

@@ -271,6 +271,50 @@ SELECT lo_create(2121);
 
 COMMENT ON LARGE OBJECT 2121 IS 'testing comments';
 
+-- Test writes on large objects in read-only transactions
+START TRANSACTION READ ONLY;
+-- INV_READ ... ok
+SELECT lo_open(2121, x'40000'::int);
+-- INV_WRITE ... error
+SELECT lo_open(2121, x'20000'::int);
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_create(42);
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_creat(42);
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_unlink(42);
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lowrite(42, 'x');
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_import(:'filename');
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_truncate(42, 0);
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_truncate64(42, 0);
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_from_bytea(0, 'x');
+ROLLBACK;
+
+START TRANSACTION READ ONLY;
+SELECT lo_put(42, 0, 'x');
+ROLLBACK;
+
 -- Clean up
 DROP TABLE lotest_stash_values;