Move description of libpqwalreceiver hooks out of the replication's README

src/backend/replication/README includes since 32bc08b a basic
description of the WAL receiver hooks available in walreceiver.h for a
module like libpqwalreceiver, but the README has never been updated to
reflect changes done to the hooks, so the contents of the README have
rotten with the time.  This commit moves the description from the README
to walreceiver.h, where it will be hard to miss that a description
update or addition is needed depending on the modifications done to the
hooks.

Each hook now includes a description of what it does in walreceiver.h,
and the replication's README mentions walreceiver.h.

Thanks also to Amit Kapila for the discussion.

Author: Michael Paquier
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/20200502024606.GA471944@paquier.xyz

Author: michaelpq

src/backend/replication/README

@@ -8,33 +8,8 @@ the primary server, receiving WAL files and sending messages, is loaded
 dynamically to avoid having to link the main server binary with libpq.
 The dynamically loaded module is in libpqwalreceiver subdirectory.
 
-The dynamically loaded module implements four functions:
-
-
-bool walrcv_connect(char *conninfo, XLogRecPtr startpoint)
-
-Establish connection to the primary, and starts streaming from 'startpoint'.
-Returns true on success.
-
-int walrcv_receive(char **buffer, pgsocket *wait_fd)
-
-Retrieve any message available without blocking through the
-connection.  If a message was successfully read, returns its
-length. If the connection is closed, returns -1.  Otherwise returns 0
-to indicate that no data is available, and sets *wait_fd to a socket
-descriptor which can be waited on before trying again.  On success, a
-pointer to the message payload is stored in *buffer. The returned
-buffer is valid until the next call to walrcv_* functions, and the
-caller should not attempt to free it.
-
-void walrcv_send(const char *buffer, int nbytes)
-
-Send a message to XLOG stream.
-
-void walrcv_disconnect(void);
-
-Disconnect.
-
+The dynamically loaded module implements a set of functions with details
+about each one of them provided in src/include/replication/walreceiver.h.
 
 This API should be considered internal at the moment, but we could open it
 up for 3rd party replacements of libpqwalreceiver in the future, allowing

src/include/replication/walreceiver.h

@@ -213,39 +213,163 @@ typedef struct WalRcvExecResult
 	TupleDesc	tupledesc;
 } WalRcvExecResult;
 
-/* libpqwalreceiver hooks */
-typedef WalReceiverConn *(*walrcv_connect_fn) (const char *conninfo, bool logical,
+/* WAL receiver - libpqwalreceiver hooks */
+
+/*
+ * walrcv_connect_fn
+ *
+ * Establish connection to a cluster.  'logical' is true if the
+ * connection is logical, and false if the connection is physical.
+ * 'appname' is a name associated to the connection, to use for example
+ * with fallback_application_name or application_name.  Returns the
+ * details about the connection established, as defined by
+ * WalReceiverConn for each WAL receiver module.  On error, NULL is
+ * returned with 'err' including the error generated.
+ */
+typedef WalReceiverConn *(*walrcv_connect_fn) (const char *conninfo,
+											   bool logical,
 											   const char *appname,
 											   char **err);
+
+/*
+ * walrcv_check_conninfo_fn
+ *
+ * Parse and validate the connection string given as of 'conninfo'.
+ */
 typedef void (*walrcv_check_conninfo_fn) (const char *conninfo);
+
+/*
+ * walrcv_get_conninfo_fn
+ *
+ * Returns a user-displayable conninfo string.  Note that any
+ * security-sensitive fields should be obfuscated.
+ */
 typedef char *(*walrcv_get_conninfo_fn) (WalReceiverConn *conn);
+
+/*
+ * walrcv_get_senderinfo_fn
+ *
+ * Provide information of the WAL sender this WAL receiver is connected
+ * to, as of 'sender_host' for the host of the sender and 'sender_port'
+ * for its port.
+ */
 typedef void (*walrcv_get_senderinfo_fn) (WalReceiverConn *conn,
 										  char **sender_host,
 										  int *sender_port);
+
+/*
+ * walrcv_identify_system_fn
+ *
+ * Run IDENTIFY_SYSTEM on the cluster connected to and validate the
+ * identity of the cluster.  Returns the system ID of the cluster
+ * connected to.  'primary_tli' is the timeline ID of the sender.
+ */
 typedef char *(*walrcv_identify_system_fn) (WalReceiverConn *conn,
 											TimeLineID *primary_tli);
+
+/*
+ * walrcv_server_version_fn
+ *
+ * Returns the version number of the cluster connected to.
+ */
 typedef int (*walrcv_server_version_fn) (WalReceiverConn *conn);
+
+/*
+ * walrcv_readtimelinehistoryfile_fn
+ *
+ * Fetch from cluster the timeline history file for timeline 'tli'.
+ * Returns the name of the timeline history file as of 'filename', its
+ * contents as of 'content' and its 'size'.
+ */
 typedef void (*walrcv_readtimelinehistoryfile_fn) (WalReceiverConn *conn,
 												   TimeLineID tli,
 												   char **filename,
-												   char **content, int *size);
+												   char **content,
+												   int *size);
+
+/*
+ * walrcv_startstreaming_fn
+ *
+ * Start streaming WAL data from given streaming options.  Returns true
+ * if the connection has switched successfully to copy-both mode and false
+ * if the server received the command and executed it successfully, but
+ * didn't switch to copy-mode.
+ */
 typedef bool (*walrcv_startstreaming_fn) (WalReceiverConn *conn,
 										  const WalRcvStreamOptions *options);
+
+/*
+ * walrcv_endstreaming_fn
+ *
+ * Stop streaming of WAL data.  Returns the next timeline ID of the cluster
+ * connected to in 'next_tli', or 0 if there was no report.
+ */
 typedef void (*walrcv_endstreaming_fn) (WalReceiverConn *conn,
 										TimeLineID *next_tli);
-typedef int (*walrcv_receive_fn) (WalReceiverConn *conn, char **buffer,
+
+/*
+ * walrcv_receive_fn
+ *
+ * Receive a message available from the WAL stream.  'buffer' is a pointer
+ * to a buffer holding the message received.  Returns the length of the data,
+ * 0 if no data is available yet ('wait_fd' is a socket descriptor which can
+ * be waited on before a retry), and -1 if the cluster ended the COPY.
+ */
+typedef int (*walrcv_receive_fn) (WalReceiverConn *conn,
+								  char **buffer,
 								  pgsocket *wait_fd);
-typedef void (*walrcv_send_fn) (WalReceiverConn *conn, const char *buffer,
+
+/*
+ * walrcv_send_fn
+ *
+ * Send a message of size 'nbytes' to the WAL stream with 'buffer' as
+ * contents.
+ */
+typedef void (*walrcv_send_fn) (WalReceiverConn *conn,
+								const char *buffer,
 								int nbytes);
+
+/*
+ * walrcv_create_slot_fn
+ *
+ * Create a new replication slot named 'slotname'.  'temporary' defines
+ * if the slot is temporary.  'snapshot_action' defines the behavior wanted
+ * for an exported snapshot (see replication protocol for more details).
+ * 'lsn' includes the LSN position at which the created slot became
+ * consistent.  Returns the name of the exported snapshot for a logical
+ * slot, or NULL for a physical slot.
+ */
 typedef char *(*walrcv_create_slot_fn) (WalReceiverConn *conn,
-										const char *slotname, bool temporary,
+										const char *slotname,
+										bool temporary,
 										CRSSnapshotAction snapshot_action,
 										XLogRecPtr *lsn);
+
+/*
+ * walrcv_get_backend_pid_fn
+ *
+ * Returns the PID of the remote backend process.
+ */
 typedef pid_t (*walrcv_get_backend_pid_fn) (WalReceiverConn *conn);
+
+/*
+ * walrcv_exec_fn
+ *
+ * Send generic queries (and commands) to the remote cluster.  'nRetTypes'
+ * is the expected number of returned attributes, and 'retTypes' an array
+ * including their type OIDs.  Returns the status of the execution and
+ * tuples if any.
+ */
 typedef WalRcvExecResult *(*walrcv_exec_fn) (WalReceiverConn *conn,
 											 const char *query,
 											 const int nRetTypes,
 											 const Oid *retTypes);
+
+/*
+ * walrcv_disconnect_fn
+ *
+ * Disconnect with the cluster.
+ */
 typedef void (*walrcv_disconnect_fn) (WalReceiverConn *conn);
 
 typedef struct WalReceiverFunctionsType