pythongh-111962: Make dtoa thread-safe in --disable-gil builds.

This avoids using the Bigint free-list in `--disable-gil` builds,
and pre-computes the needed powers of 5 during interpreter initialization.
This makes two changes to dtoa

Include/internal/pycore_dtoa.h

@@ -35,16 +35,19 @@ struct _dtoa_state {
 /* The size of the Bigint freelist */
 #define Bigint_Kmax 7
 
+/* The size of the cached powers of 5 array */
+#define Bigint_Pow5max 7
+
 #ifndef PRIVATE_MEM
 #define PRIVATE_MEM 2304
 #endif
 #define Bigint_PREALLOC_SIZE \
  ((PRIVATE_MEM+sizeof(double)-1)/sizeof(double))
 
 struct _dtoa_state {
- /* p5s is a linked list of powers of 5 of the form 5**(2**i), i >= 2 */
+ /* p5s is an array of powers of 5 of the form 5**(2**i), i >= 2 */
+ struct Bigint *p5s[Bigint_Pow5max];
  // XXX This should be freed during runtime fini.
- struct Bigint *p5s;
  struct Bigint *freelist[Bigint_Kmax+1];
  double preallocated[Bigint_PREALLOC_SIZE];
  double *preallocated_next;
@@ -57,16 +60,18 @@ struct _dtoa_state {
 #endif // !Py_USING_MEMORY_DEBUGGER
 
 
-/* These functions are used by modules compiled as C extension like math:
- they must be exported. */
-
 extern double _Py_dg_strtod(const char *str, char **ptr);
 extern char* _Py_dg_dtoa(double d, int mode, int ndigits,
  int *decpt, int *sign, char **rve);
 extern void _Py_dg_freedtoa(char *s);
 
 #endif // _PY_SHORT_FLOAT_REPR == 1
 
+
+extern PyStatus _PyDtoa_Init(PyInterpreterState *interp);
+extern void _PyDtoa_Fini(PyInterpreterState *interp);
+
+
 #ifdef __cplusplus
 }
 #endif

Python/dtoa.c

@@ -309,7 +309,7 @@ BCinfo {
 // struct Bigint is defined in pycore_dtoa.h.
 typedef struct Bigint Bigint;
 
-#ifndef Py_USING_MEMORY_DEBUGGER
+#if !defined(Py_NOGIL) && !defined(Py_USING_MEMORY_DEBUGGER)
 
 /* Memory management: memory is allocated from, and returned to, Kmax+1 pools
  of memory, where pool k (0 <= k <= Kmax) is for Bigints b with b->maxwds ==
@@ -428,7 +428,7 @@ Bfree(Bigint *v)
  }
 }
 
-#endif /* Py_USING_MEMORY_DEBUGGER */
+#endif /* !defined(Py_NOGIL) && !defined(Py_USING_MEMORY_DEBUGGER) */
 
 #define Bcopy(x,y) memcpy((char *)&x->sign, (char *)&y->sign, \
  y->wds*sizeof(Long) + 2*sizeof(int))
@@ -673,7 +673,7 @@ mult(Bigint *a, Bigint *b)
 static Bigint *
 pow5mult(Bigint *b, int k)
 {
- Bigint *b1, *p5, *p51;
+ Bigint *b1, *p5, **p5s;
  int i;
  static const int p05[3] = { 5, 25, 125 };
 
@@ -685,19 +685,12 @@ pow5mult(Bigint *b, int k)
 
  if (!(k >>= 2))
  return b;
+ assert(k < (1 << (Bigint_Pow5max)));
  PyInterpreterState *interp = _PyInterpreterState_GET();
- p5 = interp->dtoa.p5s;
- if (!p5) {
- /* first time */
- p5 = i2b(625);
- if (p5 == NULL) {
- Bfree(b);
- return NULL;
- }
- interp->dtoa.p5s = p5;
- p5->next = 0;
- }
+ p5s = interp->dtoa.p5s;
  for(;;) {
+ p5 = *p5s;
+ p5s++;
  if (k & 1) {
  b1 = mult(b, p5);
  Bfree(b);
@@ -707,17 +700,6 @@ pow5mult(Bigint *b, int k)
  }
  if (!(k >>= 1))
  break;
- p51 = p5->next;
- if (!p51) {
- p51 = mult(p5,p5);
- if (p51 == NULL) {
- Bfree(b);
- return NULL;
- }
- p51->next = 0;
- p5->next = p51;
- }
- p5 = p51;
  }
  return b;
 }
@@ -2811,3 +2793,42 @@ _Py_dg_dtoa(double dd, int mode, int ndigits,
 }
 
 #endif // _PY_SHORT_FLOAT_REPR == 1
+
+PyStatus
+_PyDtoa_Init(PyInterpreterState *interp)
+{
+#if _PY_SHORT_FLOAT_REPR == 1 && !defined(Py_USING_MEMORY_DEBUGGER)
+ Bigint **p5s = interp->dtoa.p5s;
+
+ // 5**4 = 625
+ Bigint *p5 = i2b(625);
+ if (p5 == NULL) {
+ return PyStatus_NoMemory();
+ }
+ p5s[0] = p5;
+
+ // compute 5**8, 5**16, 5**32, ..., 5**256
+ for (Py_ssize_t i = 1; i < Bigint_Pow5max; i++) {
+ p5 = mult(p5, p5);
+ if (p5 == NULL) {
+ return PyStatus_NoMemory();
+ }
+ p5s[i] = p5;
+ }
+
+#endif
+ return PyStatus_Ok();
+}
+
+void
+_PyDtoa_Fini(PyInterpreterState *interp)
+{
+#if _PY_SHORT_FLOAT_REPR == 1 && !defined(Py_USING_MEMORY_DEBUGGER)
+ Bigint **p5s = interp->dtoa.p5s;
+ for (Py_ssize_t i = 0; i < Bigint_Pow5max; i++) {
+ Bigint *p5 = p5s[i];
+ p5s[i] = NULL;
+ Bfree(p5);
+ }
+#endif
+}

Python/pylifecycle.c

@@ -825,6 +825,11 @@ pycore_interp_init(PyThreadState *tstate)
  return status;
  }
 
+ status = _PyDtoa_Init(interp);
+ if (_PyStatus_EXCEPTION(status)) {
+ return status;
+ }
+
  // The GC must be initialized before the first GC collection.
  status = _PyGC_Init(interp);
  if (_PyStatus_EXCEPTION(status)) {
@@ -1781,6 +1786,7 @@ finalize_interp_clear(PyThreadState *tstate)
  _PyXI_Fini(tstate->interp);
  _PyExc_ClearExceptionGroupType(tstate->interp);
  _Py_clear_generic_types(tstate->interp);
+ _PyDtoa_Fini(tstate->interp);
 
  /* Clear interpreter state and all thread states */
  _PyInterpreterState_Clear(tstate);