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diff --git a/src/interfaces/libpq/pqexpbuffer.h b/src/interfaces/libpq/pqexpbuffer.h
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+/*-------------------------------------------------------------------------
+ *
+ * pqexpbuffer.h
+ *	  Declarations/definitions for "PQExpBuffer" functions.
+ *
+ * PQExpBuffer provides an indefinitely-extensible string data type.
+ * It can be used to buffer either ordinary C strings (null-terminated text)
+ * or arbitrary binary data.  All storage is allocated with malloc().
+ *
+ * This module is essentially the same as the backend's StringInfo data type,
+ * but it is intended for use in frontend libpq and client applications.
+ * Thus, it does not rely on palloc() nor elog().
+ *
+ * It does rely on vsnprintf(); if configure finds that libc doesn't provide
+ * a usable vsnprintf(), then a copy of our own implementation of it will
+ * be linked into libpq.
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/interfaces/libpq/pqexpbuffer.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PQEXPBUFFER_H
+#define PQEXPBUFFER_H
+
+/*-------------------------
+ * PQExpBufferData holds information about an extensible string.
+ *		data	is the current buffer for the string (allocated with malloc).
+ *		len		is the current string length.  There is guaranteed to be
+ *				a terminating '\0' at data[len], although this is not very
+ *				useful when the string holds binary data rather than text.
+ *		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.
+ *
+ * An exception occurs if we failed to allocate enough memory for the string
+ * buffer.  In that case data points to a statically allocated empty string,
+ * and len = maxlen = 0.
+ *-------------------------
+ */
+typedef struct PQExpBufferData
+{
+	char	   *data;
+	size_t		len;
+	size_t		maxlen;
+} PQExpBufferData;
+
+typedef PQExpBufferData *PQExpBuffer;
+
+/*------------------------
+ * Test for a broken (out of memory) PQExpBuffer.
+ * When a buffer is "broken", all operations except resetting or deleting it
+ * are no-ops.
+ *------------------------
+ */
+#define PQExpBufferBroken(str)	\
+	((str) == NULL || (str)->maxlen == 0)
+
+/*------------------------
+ * Same, but for use when using a static or local PQExpBufferData struct.
+ * For that, a null-pointer test is useless and may draw compiler warnings.
+ *------------------------
+ */
+#define PQExpBufferDataBroken(buf)	\
+	((buf).maxlen == 0)
+
+/*------------------------
+ * Initial size of the data buffer in a PQExpBuffer.
+ * NB: this must be large enough to hold error messages that might
+ * be returned by PQrequestCancel().
+ *------------------------
+ */
+#define INITIAL_EXPBUFFER_SIZE	256
+
+/*------------------------
+ * There are two ways to create a PQExpBuffer object initially:
+ *
+ * PQExpBuffer stringptr = createPQExpBuffer();
+ *		Both the PQExpBufferData and the data buffer are malloc'd.
+ *
+ * PQExpBufferData string;
+ * initPQExpBuffer(&string);
+ *		The data buffer is malloc'd but the PQExpBufferData is presupplied.
+ *		This is appropriate if the PQExpBufferData is a field of another
+ *		struct.
+ *-------------------------
+ */
+
+/*------------------------
+ * createPQExpBuffer
+ * Create an empty 'PQExpBufferData' & return a pointer to it.
+ */
+extern PQExpBuffer createPQExpBuffer(void);
+
+/*------------------------
+ * initPQExpBuffer
+ * Initialize a PQExpBufferData struct (with previously undefined contents)
+ * to describe an empty string.
+ */
+extern void initPQExpBuffer(PQExpBuffer str);
+
+/*------------------------
+ * To destroy a PQExpBuffer, use either:
+ *
+ * destroyPQExpBuffer(str);
+ *		free()s both the data buffer and the PQExpBufferData.
+ *		This is the inverse of createPQExpBuffer().
+ *
+ * termPQExpBuffer(str)
+ *		free()s the data buffer but not the PQExpBufferData itself.
+ *		This is the inverse of initPQExpBuffer().
+ *
+ * NOTE: some routines build up a string using PQExpBuffer, and then
+ * release the PQExpBufferData but return the data string itself to their
+ * caller.  At that point the data string looks like a plain malloc'd
+ * string.
+ */
+extern void destroyPQExpBuffer(PQExpBuffer str);
+extern void termPQExpBuffer(PQExpBuffer str);
+
+/*------------------------
+ * resetPQExpBuffer
+ *		Reset a PQExpBuffer to empty
+ *
+ * Note: if possible, a "broken" PQExpBuffer is returned to normal.
+ */
+extern void resetPQExpBuffer(PQExpBuffer str);
+
+/*------------------------
+ * enlargePQExpBuffer
+ * Make sure there is enough space for 'needed' more bytes in the buffer
+ * ('needed' does not include the terminating null).
+ *
+ * Returns 1 if OK, 0 if failed to enlarge buffer.  (In the latter case
+ * the buffer is left in "broken" state.)
+ */
+extern int	enlargePQExpBuffer(PQExpBuffer str, size_t needed);
+
+/*------------------------
+ * printfPQExpBuffer
+ * Format text data under the control of fmt (an sprintf-like format string)
+ * and insert it into str.  More space is allocated to str if necessary.
+ * This is a convenience routine that does the same thing as
+ * resetPQExpBuffer() followed by appendPQExpBuffer().
+ */
+extern void printfPQExpBuffer(PQExpBuffer str, const char *fmt,...) pg_attribute_printf(2, 3);
+
+/*------------------------
+ * appendPQExpBuffer
+ * Format text data under the control of fmt (an sprintf-like format string)
+ * and append it to whatever is already in str.  More space is allocated
+ * to str if necessary.  This is sort of like a combination of sprintf and
+ * strcat.
+ */
+extern void appendPQExpBuffer(PQExpBuffer str, const char *fmt,...) pg_attribute_printf(2, 3);
+
+/*------------------------
+ * appendPQExpBufferStr
+ * Append the given string to a PQExpBuffer, allocating more space
+ * if necessary.
+ */
+extern void appendPQExpBufferStr(PQExpBuffer str, const char *data);
+
+/*------------------------
+ * appendPQExpBufferChar
+ * Append a single byte to str.
+ * Like appendPQExpBuffer(str, "%c", ch) but much faster.
+ */
+extern void appendPQExpBufferChar(PQExpBuffer str, char ch);
+
+/*------------------------
+ * appendBinaryPQExpBuffer
+ * Append arbitrary binary data to a PQExpBuffer, allocating more space
+ * if necessary.
+ */
+extern void appendBinaryPQExpBuffer(PQExpBuffer str,
+									const char *data, size_t datalen);
+
+#endif							/* PQEXPBUFFER_H */