libpq: Remove PQservice()

This routine has been introduced as a shortcut to be able to retrieve a
service name from an active connection, for psql.  Per discussion, and
as it is only used by psql, let's remove it to not clutter the libpq API
more than necessary.

The logic in psql is replaced by lookups of PQconninfoOption for the
active connection, instead, updated each time the variables are synced
by psql, the prompt shortcut relying on the variable synced.

Reported-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20250706161319.c1.nmisch@google.com
Backpatch-through: 18

doc/src/sgml/libpq.sgml

@@ -2740,26 +2740,6 @@ char *PQport(const PGconn *conn);
      </listitem>
     </varlistentry>
 
-    <varlistentry id="libpq-PQservice">
-     <term><function>PQservice</function><indexterm><primary>PQservice</primary></indexterm></term>
-
-     <listitem>
-      <para>
-       Returns the service of the active connection.
-
-<synopsis>
-char *PQservice(const PGconn *conn);
-</synopsis>
-      </para>
-
-      <para>
-       <xref linkend="libpq-PQservice"/> returns <symbol>NULL</symbol> if the
-       <parameter>conn</parameter> argument is <symbol>NULL</symbol>.
-       Otherwise, if there was no service provided, it returns an empty string.
-      </para>
-     </listitem>
-    </varlistentry>
-
     <varlistentry id="libpq-PQtty">
      <term><function>PQtty</function><indexterm><primary>PQtty</primary></indexterm></term>
 

src/bin/psql/command.c

@@ -4480,6 +4480,7 @@ SyncVariables(void)
 {
 	char		vbuf[32];
 	const char *server_version;
+	char	   *service_name;
 
 	/* get stuff from connection */
 	pset.encoding = PQclientEncoding(pset.db);
@@ -4489,12 +4490,16 @@ SyncVariables(void)
 	setFmtEncoding(pset.encoding);
 
 	SetVariable(pset.vars, "DBNAME", PQdb(pset.db));
-	SetVariable(pset.vars, "SERVICE", PQservice(pset.db));
 	SetVariable(pset.vars, "USER", PQuser(pset.db));
 	SetVariable(pset.vars, "HOST", PQhost(pset.db));
 	SetVariable(pset.vars, "PORT", PQport(pset.db));
 	SetVariable(pset.vars, "ENCODING", pg_encoding_to_char(pset.encoding));
 
+	service_name = get_conninfo_value("service");
+	SetVariable(pset.vars, "SERVICE", service_name);
+	if (service_name)
+		pg_free(service_name);
+
 	/* this bit should match connection_warnings(): */
 	/* Try to get full text form of version, might include "devel" etc */
 	server_version = PQparameterStatus(pset.db, "server_version");

src/bin/psql/common.c

@@ -2531,6 +2531,41 @@ session_username(void)
 		return PQuser(pset.db);
 }
 
+/*
+ * Return the value of option for keyword in the current connection.
+ *
+ * The caller is responsible for freeing the result value allocated.
+ */
+char *
+get_conninfo_value(const char *keyword)
+{
+	PQconninfoOption *opts;
+	PQconninfoOption *serviceopt = NULL;
+	char	   *res = NULL;
+
+	if (pset.db == NULL)
+		return NULL;
+
+	opts = PQconninfo(pset.db);
+	if (opts == NULL)
+		return NULL;
+
+	for (PQconninfoOption *opt = opts; opt->keyword != NULL; ++opt)
+	{
+		if (strcmp(opt->keyword, keyword) == 0)
+		{
+			serviceopt = opt;
+			break;
+		}
+	}
+
+	/* Take a copy of the value, as it is freed by PQconninfoFree(). */
+	if (serviceopt && serviceopt->val != NULL)
+		res = pg_strdup(serviceopt->val);
+	PQconninfoFree(opts);
+
+	return res;
+}
 
 /* expand_tilde
  *

src/bin/psql/common.h

@@ -39,6 +39,7 @@ extern bool SendQuery(const char *query);
 extern bool is_superuser(void);
 extern bool standard_strings(void);
 extern const char *session_username(void);
+extern char *get_conninfo_value(const char *keyword);
 
 extern void expand_tilde(char **filename);
 extern void clean_extended_state(void);

src/bin/psql/prompt.c

@@ -169,8 +169,12 @@ get_prompt(promptStatus_t status, ConditionalStack cstack)
 					break;
 					/* service name */
 				case 's':
-					if (pset.db && PQservice(pset.db))
-						strlcpy(buf, PQservice(pset.db), sizeof(buf));
+					{
+						const char *service_name = GetVariable(pset.vars, "SERVICE");
+
+						if (service_name)
+							strlcpy(buf, service_name, sizeof(buf));
+					}
 					break;
 					/* backend pid */
 				case 'p':

src/interfaces/libpq/exports.txt

@@ -205,9 +205,8 @@ PQcancelFinish            202
 PQsocketPoll              203
 PQsetChunkedRowsMode      204
 PQgetCurrentTimeUSec      205
-PQservice                 206
-PQsetAuthDataHook         207
-PQgetAuthDataHook         208
-PQdefaultAuthDataHook     209
-PQfullProtocolVersion     210
-appendPQExpBufferVA       211
+PQsetAuthDataHook         206
+PQgetAuthDataHook         207
+PQdefaultAuthDataHook     208
+PQfullProtocolVersion     209
+appendPQExpBufferVA       210

src/interfaces/libpq/fe-connect.c

@@ -7461,14 +7461,6 @@ PQdb(const PGconn *conn)
 	return conn->dbName;
 }
 
-char *
-PQservice(const PGconn *conn)
-{
-	if (!conn)
-		return NULL;
-	return conn->pgservice;
-}
-
 char *
 PQuser(const PGconn *conn)
 {

src/interfaces/libpq/libpq-fe.h

@@ -400,7 +400,6 @@ extern int	PQrequestCancel(PGconn *conn);
 
 /* Accessor functions for PGconn objects */
 extern char *PQdb(const PGconn *conn);
-extern char *PQservice(const PGconn *conn);
 extern char *PQuser(const PGconn *conn);
 extern char *PQpass(const PGconn *conn);
 extern char *PQhost(const PGconn *conn);