*: Go 1.14 support branch (#1727)

* tests: misc test fixes for go1.14

- math.go is now ambiguous due to changes to the go runtime so specify
  that we mean our own math.go in _fixtures
- go list -m requires vendor-mode to be disabled so pass '-mod=' to it
  in case user has GOFLAGS=-mod=vendor
- update version of go/packages, required to work with go 1.14 (and
  executed go mod vendor)
- Increased goroutine migration in one development version of Go 1.14
  revealed a problem with TestCheckpoints in command_test.go and
  rr_test.go. The tests were always wrong because Restart(checkpoint)
  doesn't change the current thread but we can't assume that when the
  checkpoint was taken the current goroutine was running on the same
  thread.

* goversion: update maximum supported version

* Makefile: disable testing lldb-server backend on linux with Go 1.14

There seems to be some incompatibility with lldb-server version 6.0.0
on linux and Go 1.14.

* proc/gdbserial: better handling of signals

- if multiple signals are received simultaneously propagate all of them to the
  target threads instead of only one.
- debugserver will drop an interrupt request if a target thread simultaneously
  receives a signal, handle this situation.

* dwarf/line: normalize backslashes for windows executables

Starting with Go 1.14 the compiler sometimes emits backslashes as well
as forward slashes in debug_line, normalize everything to / for
conformity with the behavior of previous versions.

* proc/native: partial support for Windows async preempt mechanism

See https://github.com/golang/go/issues/36494 for a description of why
full support for 1.14 under windows is problematic.

* proc/native: disable Go 1.14 async preemption on Windows

See https://github.com/golang/go/issues/36494

pkg/dwarf/line/line_parser.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/binary"
 	"path/filepath"
+	"strings"
 
 	"github.com/go-delve/delve/pkg/dwarf/util"
 )
@@ -34,9 +35,12 @@ type DebugLineInfo struct {
 
 	// lastMachineCache[pc] is a state machine stopped at an address after pc
 	lastMachineCache map[uint64]*StateMachine
-	
+
 	// staticBase is the address at which the executable is loaded, 0 for non-PIEs
 	staticBase uint64
+
+	// if normalizeBackslash is true all backslashes (\) will be converted into forward slashes (/)
+	normalizeBackslash bool
 }
 
 type FileEntry struct {
@@ -49,7 +53,7 @@ type FileEntry struct {
 type DebugLines []*DebugLineInfo
 
 // ParseAll parses all debug_line segments found in data
-func ParseAll(data []byte, logfn func(string, ...interface{}), staticBase uint64) DebugLines {
+func ParseAll(data []byte, logfn func(string, ...interface{}), staticBase uint64, normalizeBackslash bool) DebugLines {
 	var (
 		lines = make(DebugLines, 0)
 		buf   = bytes.NewBuffer(data)
@@ -57,7 +61,7 @@ func ParseAll(data []byte, logfn func(string, ...interface{}), staticBase uint64
 
 	// We have to parse multiple file name tables here.
 	for buf.Len() > 0 {
-		lines = append(lines, Parse("", buf, logfn, staticBase))
+		lines = append(lines, Parse("", buf, logfn, staticBase, normalizeBackslash))
 	}
 
 	return lines
@@ -65,7 +69,7 @@ func ParseAll(data []byte, logfn func(string, ...interface{}), staticBase uint64
 
 // Parse parses a single debug_line segment from buf. Compdir is the
 // DW_AT_comp_dir attribute of the associated compile unit.
-func Parse(compdir string, buf *bytes.Buffer, logfn func(string, ...interface{}), staticBase uint64) *DebugLineInfo {
+func Parse(compdir string, buf *bytes.Buffer, logfn func(string, ...interface{}), staticBase uint64, normalizeBackslash bool) *DebugLineInfo {
 	dbl := new(DebugLineInfo)
 	dbl.Logf = logfn
 	dbl.staticBase = staticBase
@@ -76,6 +80,7 @@ func Parse(compdir string, buf *bytes.Buffer, logfn func(string, ...interface{})
 
 	dbl.stateMachineCache = make(map[uint64]*StateMachine)
 	dbl.lastMachineCache = make(map[uint64]*StateMachine)
+	dbl.normalizeBackslash = normalizeBackslash
 
 	parseDebugLinePrologue(dbl, buf)
 	parseIncludeDirs(dbl, buf)
@@ -139,6 +144,10 @@ func readFileEntry(info *DebugLineInfo, buf *bytes.Buffer, exitOnEmptyPath bool)
 		return entry
 	}
 
+	if info.normalizeBackslash {
+		entry.Path = strings.Replace(entry.Path, "\\", "/", -1)
+	}
+
 	entry.DirIdx, _ = util.DecodeULEB128(buf)
 	entry.LastModTime, _ = util.DecodeULEB128(buf)
 	entry.Length, _ = util.DecodeULEB128(buf)