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
@@ -72,6 +74,23 @@ func (b bsdSysctl) Value() (float64, error) {
 		tmp64, err = unix.SysctlUint64(b.mib)
 		tmpf64 = float64(tmp64)
 	case bsdSysctlTypeStructTimeval:
+		tmpf64, err = b.getStructTimeval()
+	case bsdSysctlTypeCLong:
+		tmpf64, err = b.getCLong()
+	}
+
+	if err != nil {
+		return 0, err
+	}
+
+	if b.conversion != nil {
+		return b.conversion(tmpf64), nil
+	}
+
+	return tmpf64, nil
+}
+
+func (b bsdSysctl) getStructTimeval() (float64, error) {
 	raw, err := unix.SysctlRaw(b.mib)
 	if err != nil {
 		return 0, err
@@ -105,18 +124,34 @@ func (b bsdSysctl) Value() (float64, error) {
 	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))
+	// 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 b.conversion != nil {
-		return b.conversion(tmpf64), nil
+	if len(raw) == C.sizeof_long {
+		return float64(*(*C.long)(unsafe.Pointer(&raw[0]))), nil
 	}
 
-	return tmpf64, 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,
+	)
+
 }