Speed up readLine by using getc_unlocked

- Only on POSIX and Windows (_fgetc_nolock here) systems
- File is locked to our thread before and unlocked after readLine
- About 3 times faster in some simple tests

lib/system/sysio.nim

@@ -31,6 +31,24 @@ proc fprintf(f: File, frmt: cstring) {.importc: "fprintf",
 proc strlen(c: cstring): int {.
   importc: "strlen", header: "<string.h>", tags: [].}
 
+when defined(posix):
+  proc getc_unlocked(stream: File): cint {.importc: "getc_unlocked",
+    header: "<stdio.h>", tags: [ReadIOEffect].}
+
+  proc flockfile(stream: File) {.importc: "flockfile", header: "<stdio.h>",
+    tags: [ReadIOEffect].}
+
+  proc funlockfile(stream: File) {.importc: "funlockfile", header: "<stdio.h>",
+    tags: [ReadIOEffect].}
+elif defined(windows):
+  proc getc_unlocked(stream: File): cint {.importc: "_fgetc_nolock",
+    header: "<stdio.h>", tags: [ReadIOEffect].}
+
+  proc flockfile(stream: File) {.importc: "_lock_file", header: "<stdio.h>",
+    tags: [ReadIOEffect].}
+
+  proc funlockfile(stream: File) {.importc: "_unlock_file", header: "<stdio.h>",
+    tags: [ReadIOEffect].}
 
 # C routine that is used here:
 proc fread(buf: pointer, size, n: int, f: File): int {.
@@ -67,22 +85,40 @@ const
 proc raiseEIO(msg: string) {.noinline, noreturn.} =
   sysFatal(IOError, msg)
 
-proc readLine(f: File, line: var TaintedString): bool =
-  # of course this could be optimized a bit; but IO is slow anyway...
-  # and it was difficult to get this CORRECT with Ansi C's methods
-  setLen(line.string, 0) # reuse the buffer!
-  while true:
-    var c = fgetc(f)
-    if c < 0'i32:
-      if line.len > 0: break
-      else: return false
-    if c == 10'i32: break # LF
-    if c == 13'i32:  # CR
-      c = fgetc(f) # is the next char LF?
-      if c != 10'i32: ungetc(c, f) # no, put the character back
-      break
-    add line.string, chr(int(c))
-  result = true
+when defined(posix) or defined(windows):
+  proc readLine(f: File, line: var TaintedString): bool =
+    setLen(line.string, 0) # reuse the buffer!
+    flockfile(f)
+    while true:
+      var c = getc_unlocked(f)
+      if c < 0'i32:
+        if line.len > 0: break
+        else: return false
+      if c == 10'i32: break # LF
+      if c == 13'i32:  # CR
+        c = getc_unlocked(f) # is the next char LF?
+        if c != 10'i32: ungetc(c, f) # no, put the character back
+        break
+      add line.string, chr(int(c))
+    result = true
+    funlockfile(f)
+else:
+  proc readLine(f: File, line: var TaintedString): bool =
+    # of course this could be optimized a bit; but IO is slow anyway...
+    # and it was difficult to get this CORRECT with Ansi C's methods
+    setLen(line.string, 0) # reuse the buffer!
+    while true:
+      var c = fgetc(f)
+      if c < 0'i32:
+        if line.len > 0: break
+        else: return false
+      if c == 10'i32: break # LF
+      if c == 13'i32:  # CR
+        c = fgetc(f) # is the next char LF?
+        if c != 10'i32: ungetc(c, f) # no, put the character back
+        break
+      add line.string, chr(int(c))
+    result = true
 
 proc readLine(f: File): TaintedString =
   result = TaintedString(newStringOfCap(80))