Refactor CPU usage stats test to use go:embed

Refactor the system CPU usage testing approach for improved maintainability:

1. Extract the core CPU usage parsing logic into a new `readSystemCPUUsage`
   function that accepts an io.Reader, making it more testable and modular.

2. Use go:embed directive to embed the test data file at compile time,
   eliminating runtime file operations and making tests more reliable.

3. Simplify the test by removing global variable mocking in favor of a more
   direct approach with the new reader-based function.

4. Maintain full test coverage for the long "intr" line edge case which was
   crucial for the original bug fix, while making the test more maintainable.

This change preserves the original test behavior while improving code quality,
testability, and making the tests self-contained.

Signed-off-by: Lee Gaines <leetgaines@gmail.com>

daemon/stats_unix.go

@@ -6,6 +6,7 @@ import (
 	"bufio"
 	"context"
 	"fmt"
+	"io"
 	"os"
 	"strconv"
 	"strings"
@@ -311,30 +312,29 @@ const (
 	nanoSecondsPerSecond = 1e9
 )
 
-var procStatPath = "/proc/stat"
-
-// getSystemCPUUsage returns the host system's cpu usage in
-// nanoseconds and number of online CPUs. An error is returned
-// if the format of the underlying file does not match.
-//
-// Uses /proc/stat defined by POSIX. Looks for the cpu
-// statistics line and then sums up the first seven fields
-// provided. See `man 5 proc` for details on specific field
-// information.
+// getSystemCPUUsage reads the system's CPU usage from /proc/stat and returns
+// the total CPU usage in nanoseconds and the number of CPUs.
 func getSystemCPUUsage() (cpuUsage uint64, cpuNum uint32, _ error) {
-	f, err := os.Open(procStatPath)
+	f, err := os.Open("/proc/stat")
 	if err != nil {
 		return 0, 0, err
 	}
 	defer f.Close()
 
-	rdr := bufio.NewReaderSize(f, 1024)
+	return readSystemCPUUsage(f)
+}
+
+// readSystemCPUUsage parses CPU usage information from a reader providing
+// /proc/stat format data. It returns the total CPU usage in nanoseconds
+// and the number of CPUs.
+func readSystemCPUUsage(r io.Reader) (cpuUsage uint64, cpuNum uint32, _ error) {
+	rdr := bufio.NewReaderSize(r, 1024)
 
 	for {
 		data, isPartial, err := rdr.ReadLine()
 
 		if err != nil {
-			return 0, 0, fmt.Errorf("error scanning '%s' file: %w", procStatPath, err)
+			return 0, 0, fmt.Errorf("error scanning /proc/stat file: %w", err)
 		}
 		// Assume all cpu* records are at the start of the file, like glibc:
 		// https://github.com/bminor/glibc/blob/5d00c201b9a2da768a79ea8d5311f257871c0b43/sysdeps/unix/sysv/linux/getsysstats.c#L108-L135
@@ -358,8 +358,7 @@ func getSystemCPUUsage() (cpuUsage uint64, cpuNum uint32, _ error) {
 				}
 				totalClockTicks += v
 			}
-			cpuUsage = (totalClockTicks * nanoSecondsPerSecond) /
-				clockTicksPerSecond
+			cpuUsage = (totalClockTicks * nanoSecondsPerSecond) / clockTicksPerSecond
 		}
 		if '0' <= line[3] && line[3] <= '9' {
 			cpuNum++

daemon/stats_unix_test.go

@@ -3,28 +3,20 @@
 package daemon
 
 import (
-	"os"
-	"path/filepath"
+	_ "embed"
+	"strings"
 	"testing"
 
 	"gotest.tools/v3/assert"
+	is "gotest.tools/v3/assert/cmp"
 )
 
+//go:embed testdata/stat
+var statData string
+
 func TestGetSystemCPUUsageParsing(t *testing.T) {
-	dummyFilePath := filepath.Join("testdata", "stat")
-	expectedCpuUsage := uint64(65647090000000)
-	expectedCpuNum := uint32(128)
-
-	origStatPath := procStatPath
-	procStatPath = dummyFilePath
-	defer func() { procStatPath = origStatPath }()
-
-	_, err := os.Stat(dummyFilePath)
-	assert.NilError(t, err)
-
-	cpuUsage, cpuNum, err := getSystemCPUUsage()
-
-	assert.Equal(t, cpuUsage, expectedCpuUsage)
-	assert.Equal(t, cpuNum, expectedCpuNum)
-	assert.NilError(t, err)
+	input := strings.NewReader(statData)
+	cpuUsage, cpuNum, _ := readSystemCPUUsage(input)
+	assert.Check(t, is.Equal(cpuUsage, uint64(65647090000000)))
+	assert.Check(t, is.Equal(cpuNum, uint32(128)))
 }