kernel: remove input stream stack

Instead of pre-allocating a fixed number of TypInputFile instances on each
thread, we allocate the required storage dynamically on the stack. This
arguably makes it easier to reason about the global state of GAP.

It also enables future simplifications and improvements, e.g. further
decoupling the input file code from the line buffering and potential
optimizations resulting from that.

src/common.h

@@ -205,7 +205,7 @@ typedef const struct init_info StructInitInfo;
 *T  TypInputFile  . . . . . . . . . .  structure of an open input file, local
 **
 **  This is a forward declaration so that TypInputFile can be used in header
-**  files. The actual declaration is in io.c and is private.
+**  files. The actual declaration is in io.h.
 */
 typedef struct TypInputFile TypInputFile;
 

src/gap.c

@@ -197,14 +197,13 @@ static Obj Shell(Obj    context,
   if (!OpenOutput(outFile, FALSE))
       ErrorQuit("SHELL: can't open outfile %s",(Int)outFile,0);
 
-  if(!OpenInput(inFile))
+  TypInputFile input = { 0 };
+  if (!OpenInput(&input, inFile))
     {
       CloseOutput();
       ErrorQuit("SHELL: can't open infile %s",(Int)inFile,0);
     }
 
-  TypInputFile * input = GetCurrentInput();
-
   oldPrintObjState = SetPrintObjState(0);
 
   while ( 1 ) {
@@ -255,7 +254,7 @@ static Obj Shell(Obj    context,
     STATE(ErrorLVars) = errorLVars;
 
     /* now  read and evaluate and view one command  */
-    status = ReadEvalCommand(errorLVars, input, &evalResult, &dualSemicolon);
+    status = ReadEvalCommand(errorLVars, &input, &evalResult, &dualSemicolon);
     if (STATE(UserHasQUIT))
       break;
 
@@ -310,14 +309,14 @@ static Obj Shell(Obj    context,
 
     if (STATE(UserHasQuit))
       {
-        FlushRestOfInputLine(input);
+        FlushRestOfInputLine(&input);
         STATE(UserHasQuit) = 0;        /* quit has done its job if we are here */
       }
 
   }
 
   SetPrintObjState(oldPrintObjState);
-  CloseInput();
+  CloseInput(&input);
   CloseOutput();
   STATE(ErrorLLevel) = oldErrorLLevel;
   SetRecursionDepth(oldRecursionDepth);
@@ -402,11 +401,12 @@ int realmain( int argc, char * argv[] )
                                    read of init.g  somehow*/
     /* maybe compile in which case init.g got skipped */
     if ( SyCompilePlease ) {
-      if ( ! OpenInput(SyCompileInput) ) {
+      TypInputFile input = { 0 };
+      if ( ! OpenInput(&input, SyCompileInput) ) {
         return 1;
       }
-      func = READ_AS_FUNC();
-      if (!CloseInput()) {
+      func = READ_AS_FUNC(&input);
+      if (!CloseInput(&input)) {
           return 2;
       }
       crc  = SyGAPCRC(SyCompileInput);