Subject: [PATCH 2/3] Improve pg_ctl postmaster process check on Windows

From: Kyotaro Horiguchi <[email-masked]>
Date: Wed, 20 Sep 2023 13:16:35 +0900

Currently pg_ctl on Windows does not verify that it actually executed
a postmaster process due to lack of process ID knowledge. This can
lead to false positives in cases where another pg_ctl instance starts
a different server simultaneously.
This patch adds the capability to identify the process ID of the
launched postmaster on Windows, similar to other OS versions, ensuring
more reliable detection of concurrent server startups.
---
 src/bin/pg_ctl/pg_ctl.c | 109 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 100 insertions(+), 9 deletions(-)

diff --git a/src/bin/pg_ctl/pg_ctl.c b/src/bin/pg_ctl/pg_ctl.c
index 3ac2fcc004..ed1b0c43fc 100644
--- a/src/bin/pg_ctl/pg_ctl.c
+++ b/src/bin/pg_ctl/pg_ctl.c
@@ -132,6 +132,7 @@ static void adjust_data_dir(void);
 
 #ifdef WIN32
 #include <versionhelpers.h>
+#include <tlhelp32.h>
 static bool pgwin32_IsInstalled(SC_HANDLE);
 static char *pgwin32_CommandLine(bool);
 static void pgwin32_doRegister(void);
@@ -142,6 +143,7 @@ static void WINAPI pgwin32_ServiceMain(DWORD, LPTSTR *);
 static void pgwin32_doRunAsService(void);
 static int	CreateRestrictedProcess(char *cmd, PROCESS_INFORMATION *processInfo, bool as_service);
 static PTOKEN_PRIVILEGES GetPrivilegesToDelete(HANDLE hToken);
+static pid_t pgwin32_find_postmaster_pid(pid_t shell_pid);
 #endif
 
 static pid_t get_pgpid(bool is_status_request);
@@ -609,7 +611,11 @@ wait_for_postmaster_start(pid_t pm_pid, bool do_checkpoint)
 			/* File is complete enough for us, parse it */
 			pid_t		pmpid;
 			time_t		pmstart;
-
+#ifndef WIN32
+			pid_t		wait_pid = pm_pid;
+#else
+			pid_t		wait_pid = pgwin32_find_postmaster_pid(pm_pid);
+#endif
 			/*
 			 * Make sanity checks.  If it's for the wrong PID, or the recorded
 			 * start time is before pg_ctl started, then either we are looking
@@ -619,14 +625,8 @@ wait_for_postmaster_start(pid_t pm_pid, bool do_checkpoint)
 			 */
 			pmpid = atol(optlines[LOCK_FILE_LINE_PID - 1]);
 			pmstart = atol(optlines[LOCK_FILE_LINE_START_TIME - 1]);
-			if (pmstart >= start_time - 2 &&
-#ifndef WIN32
-				pmpid == pm_pid
-#else
-			/* Windows can only reject standalone-backend PIDs */
-				pmpid > 0
-#endif
-				)
+
+			if (pmstart >= start_time - 2 && pmpid == wait_pid)
 			{
 				/*
 				 * OK, seems to be a valid pidfile from our child.  Check the
@@ -1950,6 +1950,97 @@ GetPrivilegesToDelete(HANDLE hToken)
 
 	return tokenPrivs;
 }
+
+/*
+ * Find the PID of the launched postmaster.
+ *
+ * On Windows, the cmd.exe doesn't support the exec command. As a result, we
+ * don't directly get the postmaster's PID. This function identifies the PID of
+ * the postmaster started by the child cmd.exe.
+ *
+ * Returns the postmaster's PID. If the shell is alive but the postmaster is
+ * missing, returns 0. Otherwise terminates this command with an error.
+ *
+ * This function uses PID 0 as an invalid value, assuming the system idle
+ * process occupies it and it won't be a PID for a shell or postmaster.
+ */
+pid_t
+pgwin32_find_postmaster_pid(pid_t shell_pid)
+{
+	HANDLE hSnapshot;
+	PROCESSENTRY32 ppe;
+	pid_t pm_pid = 0;			/* abusing 0 as an invalid value */
+	bool shell_exists = false;
+	bool multiple_children = false;
+	DWORD last_error;
+
+	/* create a process snapshot */
+	hSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
+	if (hSnapshot == INVALID_HANDLE_VALUE)
+	{
+		write_stderr(_("%s: CreateToolhelp32Snapshot failed\n"),
+					 progname);
+		exit(1);
+	}
+
+	/* start iterating on the snapshot */
+	ppe.dwSize = sizeof(PROCESSENTRY32);
+	if (!Process32First(hSnapshot, &ppe))
+	{
+		write_stderr(_("%s: Process32First failed: errcode=%08lx\n"),
+					 progname, GetLastError());
+		exit(1);
+	}
+
+	/*
+	 * Iterate over the snapshot
+	 *
+	 * Check for shell existence and duplicate processes for reliability.
+	 *
+	 * The launcher shell may start other instances of cmd.exe or programs
+	 * besides postgres.exe. It's important to verify the program file name.
+	 */
+	do
+	{
+		if (ppe.th32ProcessID == shell_pid)
+			shell_exists = true;
+		else if (ppe.th32ParentProcessID == shell_pid &&
+				 strcmp("postgres.exe", ppe.szExeFile) == 0)
+		{
+			if (pm_pid != ppe.th32ProcessID && pm_pid != 0)
+				multiple_children = true;
+			pm_pid = ppe.th32ProcessID;
+		}
+	}
+	while (Process32Next(hSnapshot, &ppe));
+
+	/* avoid multiple calls primary for clarity, not out of necessity */
+	last_error = GetLastError();
+	if (last_error != ERROR_NO_MORE_FILES)
+	{
+		write_stderr(_("%s: Process32Next failed: errcode=%08lx\n"),
+					 progname, last_error);
+		exit(1);
+	}
+	CloseHandle(hSnapshot);
+
+	/* assuming the launching shell executes a single process */
+	if (multiple_children)
+	{
+		write_stderr(_("%s: launcher shell executed multiple processes\n"),
+					 progname);
+		exit(1);
+	}
+
+	/* check if the process is still alive */
+	if (!shell_exists)
+	{
+		write_stderr(_("%s: launcher shell died\n"), progname);
+		exit(1);
+	}
+
+	return pm_pid;
+}
 #endif							/* WIN32 */
 
 static void
-- 
2.39.3

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