closes bpo-39721: Fix constness of members of tok_state struct. (GH-18600)

The function PyTokenizer_FromUTF8 from Parser/tokenizer.c had a comment:

    /* XXX: constify members. */

This patch addresses that.

In the tok_state struct:
    * end and start were non-const but could be made const
    * str and input were const but should have been non-const

Changes to support this include:
    * decode_str() now returns a char * since it is allocated.
    * PyTokenizer_FromString() and PyTokenizer_FromUTF8() each creates a
        new char * for an allocate string instead of reusing the input
        const char *.
    * PyTokenizer_Get() and tok_get() now take const char ** arguments.
    * Various local vars are const or non-const accordingly.

I was able to remove five casts that cast away constness.

3 files changed, 37 insertions, 27 deletions

diff --git a/Parser/parsetok.c b/Parser/parsetok.c
index b0b1bd3..554455d 100644
--- a/Parser/parsetok.c
+++ b/Parser/parsetok.c
@@ -240,7 +240,7 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
 #endif
 
     for (;;) {
-        char *a, *b;
+        const char *a, *b;
         int type;
         size_t len;
         char *str;
@@ -371,7 +371,7 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
            buffer after parsing.  Trailing whitespace and comments
            are OK.  */
         if (err_ret->error == E_DONE && start == single_input) {
-            char *cur = tok->cur;
+            const char *cur = tok->cur;
             char c = *tok->cur;
 
             for (;;) {
diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index 630b0aa..f82b102 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -59,7 +59,9 @@ tok_new(void)
                                             sizeof(struct tok_state));
     if (tok == NULL)
         return NULL;
-    tok->buf = tok->cur = tok->end = tok->inp = tok->start = NULL;
+    tok->buf = tok->cur = tok->inp = NULL;
+    tok->start = NULL;
+    tok->end = NULL;
     tok->done = E_OK;
     tok->fp = NULL;
     tok->input = NULL;
@@ -111,7 +113,9 @@ error_ret(struct tok_state *tok) /* XXX */
     tok->decoding_erred = 1;
     if (tok->fp != NULL && tok->buf != NULL) /* see PyTokenizer_Free */
         PyMem_FREE(tok->buf);
-    tok->buf = tok->cur = tok->end = tok->inp = tok->start = NULL;
+    tok->buf = tok->cur = tok->inp = NULL;
+    tok->start = NULL;
+    tok->end = NULL;
     tok->done = E_DECODE;
     return NULL;                /* as if it were EOF */
 }
@@ -664,11 +668,11 @@ translate_newlines(const char *s, int exec_input, struct tok_state *tok) {
    Look for encoding declarations inside STR, and record them
    inside TOK.  */
 
-static const char *
+static char *
 decode_str(const char *input, int single, struct tok_state *tok)
 {
     PyObject* utf8 = NULL;
-    const char *str;
+    char *str;
     const char *s;
     const char *newl[2] = {NULL, NULL};
     int lineno = 0;
@@ -726,16 +730,18 @@ struct tok_state *
 PyTokenizer_FromString(const char *str, int exec_input)
 {
     struct tok_state *tok = tok_new();
+    char *decoded;
+
     if (tok == NULL)
         return NULL;
-    str = decode_str(str, exec_input, tok);
-    if (str == NULL) {
+    decoded = decode_str(str, exec_input, tok);
+    if (decoded == NULL) {
         PyTokenizer_Free(tok);
         return NULL;
     }
 
-    /* XXX: constify members. */
-    tok->buf = tok->cur = tok->end = tok->inp = (char*)str;
+    tok->buf = tok->cur = tok->inp = decoded;
+    tok->end = decoded;
     return tok;
 }
 
@@ -743,17 +749,18 @@ struct tok_state *
 PyTokenizer_FromUTF8(const char *str, int exec_input)
 {
     struct tok_state *tok = tok_new();
+    char *translated;
     if (tok == NULL)
         return NULL;
-    tok->input = str = translate_newlines(str, exec_input, tok);
-    if (str == NULL) {
+    tok->input = translated = translate_newlines(str, exec_input, tok);
+    if (translated == NULL) {
         PyTokenizer_Free(tok);
         return NULL;
     }
     tok->decoding_state = STATE_RAW;
     tok->read_coding_spec = 1;
     tok->enc = NULL;
-    tok->str = str;
+    tok->str = translated;
     tok->encoding = (char *)PyMem_MALLOC(6);
     if (!tok->encoding) {
         PyTokenizer_Free(tok);
@@ -761,8 +768,8 @@ PyTokenizer_FromUTF8(const char *str, int exec_input)
     }
     strcpy(tok->encoding, "utf-8");
 
-    /* XXX: constify members. */
-    tok->buf = tok->cur = tok->end = tok->inp = (char*)str;
+    tok->buf = tok->cur = tok->inp = translated;
+    tok->end = translated;
     return tok;
 }
 
@@ -812,7 +819,7 @@ PyTokenizer_Free(struct tok_state *tok)
     if (tok->fp != NULL && tok->buf != NULL)
         PyMem_FREE(tok->buf);
     if (tok->input)
-        PyMem_FREE((char *)tok->input);
+        PyMem_FREE(tok->input);
     PyMem_FREE(tok);
 }
 
@@ -1138,7 +1145,7 @@ tok_decimal_tail(struct tok_state *tok)
 /* Get next token, after space stripping etc. */
 
 static int
-tok_get(struct tok_state *tok, char **p_start, char **p_end)
+tok_get(struct tok_state *tok, const char **p_start, const char **p_end)
 {
     int c;
     int blankline, nonascii;
@@ -1321,7 +1328,7 @@ tok_get(struct tok_state *tok, char **p_start, char **p_end)
                          && ((unsigned char)ignore_end[0] >= 128 || Py_ISALNUM(ignore_end[0]))));
 
                 if (is_type_ignore) {
-                    *p_start = (char *) ignore_end;
+                    *p_start = ignore_end;
                     *p_end = tok->cur;
 
                     /* If this type ignore is the only thing on the line, consume the newline also. */
@@ -1331,7 +1338,7 @@ tok_get(struct tok_state *tok, char **p_start, char **p_end)
                     }
                     return TYPE_IGNORE;
                 } else {
-                    *p_start = (char *) type_start;  /* after type_comment_prefix */
+                    *p_start = type_start;  /* after type_comment_prefix */
                     *p_end = tok->cur;
                     return TYPE_COMMENT;
                 }
@@ -1410,7 +1417,8 @@ tok_get(struct tok_state *tok, char **p_start, char **p_end)
                    Look ahead one token to see if that is 'def'. */
 
                 struct tok_state ahead_tok;
-                char *ahead_tok_start = NULL, *ahead_tok_end = NULL;
+                const char *ahead_tok_start = NULL;
+                const char *ahead_tok_end = NULL;
                 int ahead_tok_kind;
 
                 memcpy(&ahead_tok, tok, sizeof(ahead_tok));
@@ -1798,7 +1806,7 @@ tok_get(struct tok_state *tok, char **p_start, char **p_end)
 }
 
 int
-PyTokenizer_Get(struct tok_state *tok, char **p_start, char **p_end)
+PyTokenizer_Get(struct tok_state *tok, const char **p_start, const char **p_end)
 {
     int result = tok_get(tok, p_start, p_end);
     if (tok->decoding_erred) {
@@ -1823,7 +1831,9 @@ PyTokenizer_FindEncodingFilename(int fd, PyObject *filename)
 {
     struct tok_state *tok;
     FILE *fp;
-    char *p_start =NULL , *p_end =NULL , *encoding = NULL;
+    const char *p_start = NULL;
+    const char *p_end = NULL;
+    char *encoding = NULL;
 
     fd = _Py_dup(fd);
     if (fd < 0) {
diff --git a/Parser/tokenizer.h b/Parser/tokenizer.h
index 92669bf..5660ea3 100644
--- a/Parser/tokenizer.h
+++ b/Parser/tokenizer.h
@@ -26,8 +26,8 @@ struct tok_state {
     char *buf;          /* Input buffer, or NULL; malloc'ed if fp != NULL */
     char *cur;          /* Next character in buffer */
     char *inp;          /* End of data in buffer */
-    char *end;          /* End of input buffer if buf != NULL */
-    char *start;        /* Start of current token if not NULL */
+    const char *end;    /* End of input buffer if buf != NULL */
+    const char *start;  /* Start of current token if not NULL */
     int done;           /* E_OK normally, E_EOF at EOF, otherwise error code */
     /* NB If done != E_OK, cur must be == inp!!! */
     FILE *fp;           /* Rest of input; NULL if tokenizing a string */
@@ -60,8 +60,8 @@ struct tok_state {
     PyObject *decoding_readline; /* open(...).readline */
     PyObject *decoding_buffer;
     const char* enc;        /* Encoding for the current str. */
-    const char* str;
-    const char* input; /* Tokenizer's newline translated copy of the string. */
+    char* str;
+    char* input;       /* Tokenizer's newline translated copy of the string. */
 
     int type_comments;      /* Whether to look for type comments */
 
@@ -78,7 +78,7 @@ extern struct tok_state *PyTokenizer_FromUTF8(const char *, int);
 extern struct tok_state *PyTokenizer_FromFile(FILE *, const char*,
                                               const char *, const char *);
 extern void PyTokenizer_Free(struct tok_state *);
-extern int PyTokenizer_Get(struct tok_state *, char **, char **);
+extern int PyTokenizer_Get(struct tok_state *, const char **, const char **);
 
 #define tok_dump _Py_tok_dump