fix(agent): keep Windows volume names intact when matching I/O devices (#2419)

filepath.Base is platform-dependent for a bare volume name: on Windows it
strips the "C:" specifier and returns "\", so every drive letter
normalizes to the same key. findIoDevice's exact match then returns
whichever disk.IOCounters entry the map yielded first, which registers the
root filesystem under a random drive. With EXTRA_FILESYSTEMS=D:,P: the
root disk disappears from the hub, and without it the root can be
registered under another drive's I/O device.

Normalize a bare volume specifier before taking a path base, and stop
taking a path base of the device in addPartitionRootFs, where it mangled
the value before findIoDevice could see it. registerFilesystemStats
already avoided this by only taking a base when the agent is not on
Windows.

Fixes #2417

agent/disk_test.go

@@ -1057,3 +1057,70 @@ func TestIoTimeDelta(t *testing.T) {
 
 	assert.Equal(t, uint64(0), ioTimeDelta(200, math.MaxUint32+1000))
 }
+
+func TestNormalizeDeviceName(t *testing.T) {
+	// A Windows volume name is not a path element, so every spelling of the
+	// same drive has to normalize to the same key. filepath.Base cannot do
+	// this: on Windows it strips the "C:" specifier and returns "\", which
+	// collapses every drive letter onto one key (#2417).
+	for _, spelling := range []string{"C:", `C:\`, "C:/", `C:\\`} {
+		assert.Equal(t, "C:", normalizeDeviceName(spelling), "spelling %q", spelling)
+	}
+	// Case is left to the caller, as it already is for Linux device names.
+	assert.Equal(t, "d:", normalizeDeviceName("d:"))
+	assert.Equal(t, "c:", normalizeDeviceName(" c: "))
+
+	// Non-volume inputs keep using filepath.Base.
+	assert.Equal(t, "sda1", normalizeDeviceName("/dev/sda1"))
+	assert.Equal(t, "sda1", normalizeDeviceName("/dev/sda1/"))
+	assert.Equal(t, "nvme0n1p2", normalizeDeviceName("  /dev/nvme0n1p2  "))
+	assert.Equal(t, "", normalizeDeviceName("."))
+	assert.Equal(t, "", normalizeDeviceName("   "))
+
+	// A drive-relative path is a path, not a volume.
+	assert.Equal(t, `C:data`, normalizeDeviceName(`C:data`))
+}
+
+func TestFindIoDeviceWindowsVolumeNames(t *testing.T) {
+	// Every drive normalizes to a distinct key, so the root drive resolves
+	// exactly instead of to whichever counter the map yielded first (#2417).
+	ioCounters := map[string]disk.IOCountersStat{
+		"C:": {Name: "C:", ReadBytes: 10, WriteBytes: 10},
+		"D:": {Name: "D:", ReadBytes: 20, WriteBytes: 20},
+		"P:": {Name: "P:", ReadBytes: 30, WriteBytes: 30},
+	}
+
+	for i := 0; i < 32; i++ {
+		device, ok := findIoDevice("C:", ioCounters)
+		assert.True(t, ok)
+		assert.Equal(t, "C:", device)
+	}
+
+	// The drive may arrive with a trailing separator, as a mount point does.
+	device, ok := findIoDevice(`C:\`, ioCounters)
+	assert.True(t, ok)
+	assert.Equal(t, "C:", device)
+}
+
+func TestAddPartitionRootFsWindowsDrive(t *testing.T) {
+	agent := &Agent{fsStats: make(map[string]*system.FsStats)}
+	discovery := diskDiscovery{
+		agent: agent,
+		ctx: fsRegistrationContext{
+			isWindows: true,
+			diskIoCounters: map[string]disk.IOCountersStat{
+				"C:": {Name: "C:"},
+				"D:": {Name: "D:"},
+				"P:": {Name: "P:"},
+			},
+		},
+	}
+
+	ok := discovery.addPartitionRootFs("C:", `C:\`)
+
+	assert.True(t, ok)
+	assert.Len(t, agent.fsStats, 1)
+	stats, exists := agent.fsStats["C:"]
+	assert.True(t, exists)
+	assert.True(t, stats.Root)
+}