[MM-67978] Add open file descriptor count to support packet diagnostics (#35834)

* MM-67978: Add open file descriptor count to support packet diagnostics

Add OpenFileDescriptors and MaxFileDescriptors fields to
SupportPacketDiagnostics.Server. On Linux the open count is read from
/proc/self/fd; on macOS from /dev/fd. Both platforms use
syscall.Getrlimit(RLIMIT_NOFILE) for the soft limit. Unsupported
platforms (e.g. Windows) return -1 for both fields.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

* MM-67978: Move OpenFileDescriptors/MaxFileDescriptors after TotalMemoryMB

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

* fix: guard int64 overflow in getMaxFileDescriptors and update tests for unsupported platforms

Add overflow check before casting rlimit.Cur (uint64) to int64 in
getMaxFileDescriptors on Linux. Update fd_test.go and
support_packet_test.go assertions to accept -1 as a valid sentinel for
unsupported platforms instead of requiring a strictly positive value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* revert: remove unrelated test refactoring from support_packet_test.go

Restore the three unrelated changes that crept into this branch:
- SetLogRootPathOverride instead of t.Setenv("MM_LOG_PATH", ...)
- installTypeOverride field instead of t.Setenv(envVarInstallType, ...)
- Restore explanatory comment in TestGetSanitizedConfigFile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

server/channels/app/platform/fd_darwin.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+//go:build darwin
+
+package platform
+
+import (
+	"os"
+	"syscall"
+)
+
+// getOpenFileDescriptors returns the number of open file descriptors for the current process.
+func getOpenFileDescriptors() (int64, error) {
+	entries, err := os.ReadDir("/dev/fd")
+	if err != nil {
+		return -1, err
+	}
+	// Subtract 1 because the ReadDir call itself opens a file descriptor that appears in the listing.
+	return max(int64(len(entries))-1, 0), nil
+}
+
+// getMaxFileDescriptors returns the soft file descriptor limit for the current process.
+func getMaxFileDescriptors() (int64, error) {
+	var rlimit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlimit); err != nil {
+		return -1, err
+	}
+	return int64(rlimit.Cur), nil
+}

server/channels/app/platform/fd_linux.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+//go:build linux
+
+package platform
+
+import (
+	"fmt"
+	"math"
+	"os"
+	"syscall"
+)
+
+// getOpenFileDescriptors returns the number of open file descriptors for the current process.
+func getOpenFileDescriptors() (int64, error) {
+	entries, err := os.ReadDir("/proc/self/fd")
+	if err != nil {
+		return -1, err
+	}
+	// Subtract 1 because the ReadDir call itself opens a file descriptor that appears in the listing.
+	return max(int64(len(entries))-1, 0), nil
+}
+
+// getMaxFileDescriptors returns the soft file descriptor limit for the current process.
+func getMaxFileDescriptors() (int64, error) {
+	var rlimit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlimit); err != nil {
+		return -1, err
+	}
+	if rlimit.Cur > math.MaxInt64 {
+		return -1, fmt.Errorf("rlimit.Cur %d overflows int64", rlimit.Cur)
+	}
+	return int64(rlimit.Cur), nil
+}

server/channels/app/platform/fd_other.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+//go:build !linux && !darwin
+
+package platform
+
+// getOpenFileDescriptors returns -1 on unsupported platforms.
+func getOpenFileDescriptors() (int64, error) {
+	return -1, nil
+}
+
+// getMaxFileDescriptors returns -1 on unsupported platforms.
+func getMaxFileDescriptors() (int64, error) {
+	return -1, nil
+}

server/channels/app/platform/fd_test.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package platform
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetOpenFileDescriptors(t *testing.T) {
+	count, err := getOpenFileDescriptors()
+	require.NoError(t, err)
+	if count == -1 {
+		return
+	}
+	assert.Positive(t, count)
+}
+
+func TestGetMaxFileDescriptors(t *testing.T) {
+	maxFDs, err := getMaxFileDescriptors()
+	require.NoError(t, err)
+	// -1 means unsupported platform; otherwise should be positive
+	assert.True(t, maxFDs == -1 || maxFDs > 0, "maxFDs should be -1 (unsupported) or positive, got %d", maxFDs)
+}

server/channels/app/platform/support_packet.go

@@ -118,6 +118,14 @@ func (ps *PlatformService) getSupportPacketDiagnostics(rctx request.CTX) (*model
 		installationType = unknownDataPoint
 	}
 	d.Server.InstallationType = installationType
+	d.Server.OpenFileDescriptors, err = getOpenFileDescriptors()
+	if err != nil {
+		rErr = multierror.Append(rErr, errors.Wrap(err, "error while getting open file descriptor count"))
+	}
+	d.Server.MaxFileDescriptors, err = getMaxFileDescriptors()
+	if err != nil {
+		rErr = multierror.Append(rErr, errors.Wrap(err, "error while getting max file descriptor limit"))
+	}
 	d.Server.ProcessID = os.Getpid()
 
 	/* Config */

server/channels/app/platform/support_packet_test.go

@@ -210,6 +210,8 @@ func TestGetSupportPacketDiagnostics(t *testing.T) {
 		assert.Equal(t, "docker", d.Server.InstallationType)
 		assert.Positive(t, d.Server.CPUCores)
 		assert.Positive(t, d.Server.TotalMemoryMB)
+		assert.True(t, d.Server.OpenFileDescriptors == -1 || d.Server.OpenFileDescriptors > 0, "OpenFileDescriptors should be -1 (unsupported) or positive, got %d", d.Server.OpenFileDescriptors)
+		assert.True(t, d.Server.MaxFileDescriptors == -1 || d.Server.MaxFileDescriptors > 0, "MaxFileDescriptors should be -1 (unsupported) or positive, got %d", d.Server.MaxFileDescriptors)
 		assert.Positive(t, d.Server.ProcessID)
 
 		/* Config */

server/public/model/support_packet.go

@@ -25,16 +25,18 @@ type SupportPacketDiagnostics struct {
 	} `yaml:"license"`
 
 	Server struct {
-		OS               string `yaml:"os"`
-		Architecture     string `yaml:"architecture"`
-		CPUCores         int    `yaml:"cpu_cores"`
-		TotalMemoryMB    uint64 `yaml:"total_memory_mb"`
-		Hostname         string `yaml:"hostname"`
-		ProcessID        int    `yaml:"process_id"`
-		Version          string `yaml:"version"`
-		BuildHash        string `yaml:"build_hash"`
-		GoVersion        string `yaml:"go_version"`
-		InstallationType string `yaml:"installation_type"`
+		OS                  string `yaml:"os"`
+		Architecture        string `yaml:"architecture"`
+		CPUCores            int    `yaml:"cpu_cores"`
+		TotalMemoryMB       uint64 `yaml:"total_memory_mb"`
+		OpenFileDescriptors int64  `yaml:"open_file_descriptors"`
+		MaxFileDescriptors  int64  `yaml:"max_file_descriptors"`
+		Hostname            string `yaml:"hostname"`
+		ProcessID           int    `yaml:"process_id"`
+		Version             string `yaml:"version"`
+		BuildHash           string `yaml:"build_hash"`
+		GoVersion           string `yaml:"go_version"`
+		InstallationType    string `yaml:"installation_type"`
 	} `yaml:"server"`
 
 	Config struct {