fix #9634 don't crash on execCmdEx/readLine when inside gdb/lldb (#13232)

* fix #9634 debugging a program using execCmdEx now works

* only apply EINTR to c_gets for now

This reverts commit c0f5305b5a0b46983dfd27e3d77ecbf4f8744dcc.

lib/system/io.nim

@@ -145,6 +145,7 @@ proc strerror(errnum: cint): cstring {.importc, header: "<string.h>".}
 when not defined(NimScript):
   var
     errno {.importc, header: "<errno.h>".}: cint ## error variable
+    EINTR {.importc: "EINTR", header: "<errno.h>".}: cint
 
 proc checkErr(f: File) =
   when not defined(NimScript):
@@ -319,8 +320,20 @@ proc readLine*(f: File, line: var TaintedString): bool {.tags: [ReadIOEffect],
     # fgets doesn't append an \L
     for i in 0..<sp: line.string[pos+i] = '\L'
 
-    var fgetsSuccess = c_fgets(addr line.string[pos], sp.cint, f) != nil
-    if not fgetsSuccess: checkErr(f)
+    var fgetsSuccess: bool
+    while true:
+      # fixes #9634; this pattern may need to be abstracted as a template if reused;
+      # likely other io procs need this for correctness.
+      fgetsSuccess = c_fgets(addr line.string[pos], sp.cint, f) != nil
+      if fgetsSuccess: break
+      when not defined(NimScript):
+        if errno == EINTR:
+          errno = 0
+          c_clearerr(f)
+          continue
+      checkErr(f)
+      break
+
     let m = c_memchr(addr line.string[pos], '\L'.ord, cast[csize_t](sp))
     if m != nil:
       # \l found: Could be our own or the one by fgets, in any case, we're done