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

4 files changed, 48 insertions, 3 deletions

diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index 9fcd2db8326..0a55901b14e 100644
--- a/src/bin/psql/command.c
+++ b/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");
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index d2c0a49c46c..cd329ade12b 100644
--- a/src/bin/psql/common.c
+++ b/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
  *
diff --git a/src/bin/psql/common.h b/src/bin/psql/common.h
index 7f1a23de1e8..64762ab9817 100644
--- a/src/bin/psql/common.h
+++ b/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);
diff --git a/src/bin/psql/prompt.c b/src/bin/psql/prompt.c
index 3aa7d2d06c8..b08d7328fbf 100644
--- a/src/bin/psql/prompt.c
+++ b/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':