Correct buffer_bytes > INT_MAX on BSD/amd64. (#712)

* Correct buffer_bytes > INT_MAX on BSD/amd64.

The sysctl vfs.bufspace returns either an int or a long, depending on
the value.  Large values of vfs.bufspace will result in error messages
like:

  couldn't get meminfo: cannot allocate memory

This will detect the returned data type, and cast appropriately.

* Added explicit length checks per feedback.

* Flatten Value() to make it easier to read.

* Simplify per feedback.

* Fix style.

* Doc updates.

collector/meminfo_bsd.go

@@ -39,7 +39,7 @@ func (c *meminfoCollector) getMemInfo() (map[string]float64, error) {
 		{name: "inactive_bytes", mib: "vm.stats.vm.v_inactive_count", conversion: fromPage},
 		{name: "wired_bytes", mib: "vm.stats.vm.v_wire_count", conversion: fromPage},
 		{name: "cache_bytes", mib: "vm.stats.vm.v_cache_count", conversion: fromPage},
-		{name: "buffer_bytes", mib: "vfs.bufspace"},
+		{name: "buffer_bytes", mib: "vfs.bufspace", dataType: bsdSysctlTypeCLong},
 		{name: "free_bytes", mib: "vm.stats.vm.v_free_count", conversion: fromPage},
 		{name: "size_bytes", mib: "vm.stats.vm.v_page_count", conversion: fromPage},
 		{name: "swap_in_bytes_total", mib: "vm.stats.vm.v_swappgsin", conversion: fromPage},

collector/sysctl_bsd.go

@@ -35,9 +35,11 @@ const (
 	bsdSysctlTypeUint32 bsdSysctlType = iota
 	bsdSysctlTypeUint64
 	bsdSysctlTypeStructTimeval
+	bsdSysctlTypeCLong
 )
 
-// Contains all the info needed to map a single bsd-sysctl to a prometheus value.
+// Contains all the info needed to map a single bsd-sysctl to a prometheus
+// value.
 type bsdSysctl struct {
 	// Prometheus name
 	name string
@@ -72,42 +74,9 @@ func (b bsdSysctl) Value() (float64, error) {
 		tmp64, err = unix.SysctlUint64(b.mib)
 		tmpf64 = float64(tmp64)
 	case bsdSysctlTypeStructTimeval:
-		raw, err := unix.SysctlRaw(b.mib)
-		if err != nil {
-			return 0, err
-		}
-
-		/*
-		 * From 10.3-RELEASE sources:
-		 *
-		 * /usr/include/sys/_timeval.h:47
-		 *  time_t      tv_sec
-		 *  suseconds_t tv_usec
-		 *
-		 * /usr/include/sys/_types.h:60
-		 *  long __suseconds_t
-		 *
-		 * ... architecture dependent, via #ifdef:
-		 *  typedef __int64_t __time_t;
-		 *  typedef __int32_t __time_t;
-		 */
-		if len(raw) != (C.sizeof_time_t + C.sizeof_suseconds_t) {
-			// Shouldn't get here, unless the ABI changes...
-			return 0, fmt.Errorf(
-				"length of bytes received from sysctl (%d) does not match expected bytes (%d)",
-				len(raw),
-				C.sizeof_time_t+C.sizeof_suseconds_t,
-			)
-		}
-
-		secondsUp := unsafe.Pointer(&raw[0])
-		susecondsUp := uintptr(secondsUp) + C.sizeof_time_t
-		unix := float64(*(*C.time_t)(secondsUp))
-		usec := float64(*(*C.suseconds_t)(unsafe.Pointer(susecondsUp)))
-
-		// This conversion maintains the usec precision.  Using
-		// the time package did not.
-		tmpf64 = unix + (usec / float64(1000*1000))
+		tmpf64, err = b.getStructTimeval()
+	case bsdSysctlTypeCLong:
+		tmpf64, err = b.getCLong()
 	}
 
 	if err != nil {
@@ -120,3 +89,69 @@ func (b bsdSysctl) Value() (float64, error) {
 
 	return tmpf64, nil
 }
+
+func (b bsdSysctl) getStructTimeval() (float64, error) {
+	raw, err := unix.SysctlRaw(b.mib)
+	if err != nil {
+		return 0, err
+	}
+
+	/*
+	 * From 10.3-RELEASE sources:
+	 *
+	 * /usr/include/sys/_timeval.h:47
+	 *  time_t      tv_sec
+	 *  suseconds_t tv_usec
+	 *
+	 * /usr/include/sys/_types.h:60
+	 *  long __suseconds_t
+	 *
+	 * ... architecture dependent, via #ifdef:
+	 *  typedef __int64_t __time_t;
+	 *  typedef __int32_t __time_t;
+	 */
+	if len(raw) != (C.sizeof_time_t + C.sizeof_suseconds_t) {
+		// Shouldn't get here, unless the ABI changes...
+		return 0, fmt.Errorf(
+			"length of bytes received from sysctl (%d) does not match expected bytes (%d)",
+			len(raw),
+			C.sizeof_time_t+C.sizeof_suseconds_t,
+		)
+	}
+
+	secondsUp := unsafe.Pointer(&raw[0])
+	susecondsUp := uintptr(secondsUp) + C.sizeof_time_t
+	unix := float64(*(*C.time_t)(secondsUp))
+	usec := float64(*(*C.suseconds_t)(unsafe.Pointer(susecondsUp)))
+
+	// This conversion maintains the usec precision.  Using the time
+	// package did not.
+	return (unix + (usec / float64(1000*1000))), nil
+}
+
+func (b bsdSysctl) getCLong() (float64, error) {
+	raw, err := unix.SysctlRaw(b.mib)
+	if err != nil {
+		return 0, err
+	}
+
+	if len(raw) == C.sizeof_long {
+		return float64(*(*C.long)(unsafe.Pointer(&raw[0]))), nil
+	}
+
+	if len(raw) == C.sizeof_int {
+		// This is valid for at least vfs.bufspace, and the default
+		// long handler - which can clamp longs to 32-bits:
+		//   https://github.com/freebsd/freebsd/blob/releng/10.3/sys/kern/vfs_bio.c#L338
+		//   https://github.com/freebsd/freebsd/blob/releng/10.3/sys/kern/kern_sysctl.c#L1062
+		return float64(*(*C.int)(unsafe.Pointer(&raw[0]))), nil
+	}
+
+	return 0, fmt.Errorf(
+		"length of bytes received from sysctl (%d) does not match expected bytes (long: %d), (int: %d)",
+		len(raw),
+		C.sizeof_long,
+		C.sizeof_int,
+	)
+
+}