On Wed, 11 Oct 2023 at 08:52, Tom Lane <[email protected]> wrote:
>
> David Rowley <[email protected]> writes:
> > I've attached a slightly more worked on patch that makes maxlen == 0
> > mean read-only. Unsure if a macro is worthwhile there or not.
>
> A few thoughts:
Thank you for the review.
I spent more time on this and did end up with 2 new init functions as
you mentioned. One for strictly read-only (initReadOnlyStringInfo),
which cannot be appended to, and as you mentioned, another
(initStringInfoFromString) which can accept a palloc'd buffer which
becomes managed by the stringinfo code. I know these names aren't
exactly as you mentioned. I'm open to adjusting still.
This means I got rid of the read-only conversion code in
enlargeStringInfo(). I didn't do anything to try to handle buffer
enlargement more efficiently in enlargeStringInfo() for the case where
initStringInfoFromString sets maxlen to some non-power-of-2. The
doubling code seems like it'll work ok without power-of-2 values,
it'll just end up calling repalloc() with non-power-of-2 values.
I did also wonder if resetStringInfo() would have any business
touching the existing buffer in a read-only StringInfo and came to the
conclusion that it wouldn't be very read-only if we allowed
resetStringInfo() to do its thing on it. I added an Assert to fail if
resetStringInfo() receives a read-only StringInfo.
Also, since it's still being discussed, I left out the adjustment to
LogicalParallelApplyLoop(). That also allows the tests to pass
without the failing Assert that was checking for the NUL terminator.
David
diff --git a/src/backend/replication/logical/proto.c
b/src/backend/replication/logical/proto.c
index d52c8963eb..ce9d5b4059 100644
--- a/src/backend/replication/logical/proto.c
+++ b/src/backend/replication/logical/proto.c
@@ -879,6 +879,7 @@ logicalrep_read_tuple(StringInfo in, LogicalRepTupleData
*tuple)
/* Read the data */
for (i = 0; i < natts; i++)
{
+ char *buff;
char kind;
int len;
StringInfo value = &tuple->colvalues[i];
@@ -899,19 +900,16 @@ logicalrep_read_tuple(StringInfo in, LogicalRepTupleData
*tuple)
len = pq_getmsgint(in, 4); /* read length
*/
/* and data */
- value->data = palloc(len + 1);
- pq_copymsgbytes(in, value->data, len);
+ buff = palloc(len + 1);
+ pq_copymsgbytes(in, buff, len);
/*
* Not strictly necessary for
LOGICALREP_COLUMN_BINARY, but
* per StringInfo practice.
*/
- value->data[len] = '\0';
+ buff[len] = '\0';
- /* make StringInfo fully valid */
- value->len = len;
- value->cursor = 0;
- value->maxlen = len;
+ initStringInfoFromString(value, buff, len);
break;
default:
elog(ERROR, "unrecognized data representation
type '%c'", kind);
diff --git a/src/backend/replication/logical/worker.c
b/src/backend/replication/logical/worker.c
index 597947410f..b574188d70 100644
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -3582,10 +3582,7 @@ LogicalRepApplyLoop(XLogRecPtr last_received)
/* Ensure we are reading the data into
our memory context. */
MemoryContextSwitchTo(ApplyMessageContext);
- s.data = buf;
- s.len = len;
- s.cursor = 0;
- s.maxlen = -1;
+ initReadOnlyStringInfo(&s, buf, len);
c = pq_getmsgbyte(&s);
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index f3c9f1f9ba..94b963d6e6 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -1816,23 +1816,19 @@ exec_bind_message(StringInfo input_message)
if (!isNull)
{
- const char *pvalue =
pq_getmsgbytes(input_message, plength);
+ char *pvalue;
/*
- * Rather than copying data around, we just set
up a phony
- * StringInfo pointing to the correct portion
of the message
- * buffer. We assume we can scribble on the
message buffer so
- * as to maintain the convention that
StringInfos have a
- * trailing null. This is grotty but is a big
win when
- * dealing with very large parameter strings.
+ * Rather than copying data around, we just
initialize a StringInfo
+ * pointing to the correct portion of the
message buffer. We assume we
+ * can scribble on the message buffer so as to
maintain the convention
+ * that StringInfos have a trailing null. This
is grotty but is a big win
+ * when dealing with very large parameter
strings.
*/
- pbuf.data = unconstify(char *, pvalue);
- pbuf.maxlen = plength + 1;
- pbuf.len = plength;
- pbuf.cursor = 0;
-
- csave = pbuf.data[plength];
- pbuf.data[plength] = '\0';
+ pvalue = unconstify(char *,
pq_getmsgbytes(input_message, plength));
+ csave = pvalue[plength];
+ pvalue[plength] = '\0';
+ initReadOnlyStringInfo(&pbuf, pvalue, plength);
}
else
{
diff --git a/src/backend/utils/adt/rowtypes.c b/src/backend/utils/adt/rowtypes.c
index ad176651d8..c7ee263439 100644
--- a/src/backend/utils/adt/rowtypes.c
+++ b/src/backend/utils/adt/rowtypes.c
@@ -623,21 +623,18 @@ record_recv(PG_FUNCTION_ARGS)
}
else
{
+ char *strbuff;
+
/*
- * Rather than copying data around, we just set up a
phony
- * StringInfo pointing to the correct portion of the
input buffer.
- * We assume we can scribble on the input buffer so as
to maintain
+ * Initalize a new StringInfo using the correct portion
of the input
+ * buffer. We assume we can scribble on the input
buffer so as to maintain
* the convention that StringInfos have a trailing null.
*/
- item_buf.data = &buf->data[buf->cursor];
- item_buf.maxlen = itemlen + 1;
- item_buf.len = itemlen;
- item_buf.cursor = 0;
-
+ strbuff = &buf->data[buf->cursor];
buf->cursor += itemlen;
-
csave = buf->data[buf->cursor];
buf->data[buf->cursor] = '\0';
+ initReadOnlyStringInfo(&item_buf, strbuff, itemlen);
bufptr = &item_buf;
nulls[i] = false;
diff --git a/src/common/stringinfo.c b/src/common/stringinfo.c
index 05b22b5c53..6897c78d9f 100644
--- a/src/common/stringinfo.c
+++ b/src/common/stringinfo.c
@@ -30,6 +30,7 @@
#endif
#include "lib/stringinfo.h"
+#include "port/pg_bitutils.h"
/*
@@ -74,6 +75,9 @@ initStringInfo(StringInfo str)
void
resetStringInfo(StringInfo str)
{
+ /* Don't allow resets of read-only StringInfos */
+ Assert(str->maxlen != 0);
+
str->data[0] = '\0';
str->len = 0;
str->cursor = 0;
@@ -284,6 +288,9 @@ enlargeStringInfo(StringInfo str, int needed)
{
int newlen;
+ /* Validate this is not a read-only StringInfo */
+ Assert(str->maxlen != 0);
+
/*
* Guard against out-of-range "needed" values. Without this, we can get
* an overflow or infinite loop in the following.
diff --git a/src/include/lib/stringinfo.h b/src/include/lib/stringinfo.h
index 36a416f8e0..3bd605d528 100644
--- a/src/include/lib/stringinfo.h
+++ b/src/include/lib/stringinfo.h
@@ -27,10 +27,18 @@
* maxlen is the allocated size in bytes of 'data', i.e. the
maximum
* string size (including the terminating '\0'
char) that we can
* currently store in 'data' without having to
reallocate
- * more space. We must always have maxlen > len.
- * cursor is initialized to zero by makeStringInfo or
initStringInfo,
- * but is not otherwise touched by the
stringinfo.c routines.
- * Some routines use it to scan through a
StringInfo.
+ * more space. We must always have maxlen > len,
except
+ * in the read-only case described below.
+ * cursor is initialized to zero by makeStringInfo,
initStringInfo,
+ * initReadOnlyStringInfo and
initStringInfoFromString but is not
+ * otherwise touched by the stringinfo.c routines.
Some routines
+ * use it to scan through a StringInfo.
+ *
+ * As a special case, a StringInfoData can be initialized with a read-only
+ * string buffer. In this case "data" does not necessarily point at a
+ * palloc'd chunk, and management of the buffer storage is the caller's
+ * responsibility. maxlen is set to zero to indicate that this is the case.
+ * Read-only StringInfoDatas cannot be appended to.
*-------------------------
*/
typedef struct StringInfoData
@@ -45,7 +53,7 @@ typedef StringInfoData *StringInfo;
/*------------------------
- * There are two ways to create a StringInfo object initially:
+ * There are four ways to create a StringInfo object initially:
*
* StringInfo stringptr = makeStringInfo();
* Both the StringInfoData and the data buffer are palloc'd.
@@ -56,8 +64,30 @@ typedef StringInfoData *StringInfo;
* This is the easiest approach for a StringInfo object that will
* only live as long as the current routine.
*
+ * StringInfoData string;
+ * initReadOnlyStringInfo(&string, existingbuf, len);
+ * The StringInfoData's data field is set to point directly to the
+ * existing buffer and the StringInfoData's len is set to the
given len.
+ * The given buffer can point to memory that's not managed by
palloc or
+ * is pointing part way through a palloc'd chunk. The maxlen
field is
+ * set to 0. A read-only StringInfo cannot be appended to using
any of
+ * the appendStringInfo functions or reset with resetStringInfo().
The
+ * given buffer must be NUL-terminated.
+ *
+ * StringInfoData string;
+ * initStringInfoFromString(&string, palloced_buf, len);
+ * The StringInfoData's data field is set to point directly to the
given
+ * buffer and the StringInfoData's len is set to the given len.
This
+ * method of initialization is useful when the buffer already
exists.
+ * StringInfos initialized this way can be appended to using the
+ * appendStringInfo functions and reset with resetStringInfo().
The
+ * given buffer must be NUL-terminated.
+ *
* To destroy a StringInfo, pfree() the data buffer, and then pfree() the
* StringInfoData if it was palloc'd. There's no special support for this.
+ * However, if the StringInfo was initialized using initReadOnlyStringInfo()
+ * then the caller will need to consider if it is safe to pfree the data
+ * buffer.
*
* NOTE: some routines build up a string using StringInfo, and then
* release the StringInfoData but return the data string itself to their
@@ -79,6 +109,48 @@ extern StringInfo makeStringInfo(void);
*/
extern void initStringInfo(StringInfo str);
+/*------------------------
+ * initReadOnlyStringInfo
+ * Initialize a StringInfoData struct from an existing string without copying
+ * the string. The caller is responsible for ensuring the given string
+ * remains valid as long as the StringInfoData does. The given string must be
+ * NUL terminated at 'len' bytes. Calls to this are used in performance
+ * critical locations where allocating a new buffer and copying would be too
+ * costly. Read-only StringInfoData's may not be appended to using any of the
+ * appendStringInfo functions or reset with resetStringInfo().
+ *
+ * 'data' does not need to point directly to a palloc'd chunk of memory.
+ */
+static inline void
+initReadOnlyStringInfo(StringInfo str, char *data, int len)
+{
+ Assert(data[len] == '\0');
+
+ str->data = data;
+ str->maxlen = 0; /* read-only */
+ str->len = len;
+ str->cursor = 0;
+}
+
+/*------------------------
+ * initStringInfoFromString
+ * Initialize a StringInfoData struct from an existing string without copying
+ * the string. 'data' must be a valid palloc'd chunk of memory that can have
+ * repalloc() called should more space be required during a call to any of the
+ * appendStringInfo functions.
+ *
+ * The given string must be NUL terminated at 'len' bytes.
+ */
+static inline void
+initStringInfoFromString(StringInfo str, char *data, int len)
+{
+ Assert(data[len] == '\0');
+
+ str->data = data;
+ str->maxlen = str->len = len;
+ str->cursor = 0;
+}
+
/*------------------------
* resetStringInfo
* Clears the current content of the StringInfo, if any. The
