gh-88116: Avoid undefined behavior when decoding varints in code objects (GH-94375)

(cherry picked from commit c485ec0)

Co-authored-by: Pablo Galindo Salgado <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2022-06-28-12-41-17.gh-issue-88116.A7fEl_.rst

@@ -0,0 +1,2 @@
+Fix an issue when reading line numbers from code objects if the encoded line
+numbers are close to ``INT_MIN``. Patch by Pablo Galindo

Objects/codeobject.c

@@ -346,9 +346,9 @@ init_code(PyCodeObject *co, struct _PyCodeConstructor *con)
 static int
 scan_varint(const uint8_t *ptr)
 {
-    int read = *ptr++;
-    int val = read & 63;
-    int shift = 0;
+    unsigned int read = *ptr++;
+    unsigned int val = read & 63;
+    unsigned int shift = 0;
     while (read & 64) {
         read = *ptr++;
         shift += 6;
@@ -360,7 +360,7 @@ scan_varint(const uint8_t *ptr)
 static int
 scan_signed_varint(const uint8_t *ptr)
 {
-    int uval = scan_varint(ptr);
+    unsigned int uval = scan_varint(ptr);
     if (uval & 1) {
         return -(int)(uval >> 1);
     }
@@ -839,9 +839,9 @@ read_byte(PyCodeAddressRange *bounds)
 static int
 read_varint(PyCodeAddressRange *bounds)
 {
-    int read = read_byte(bounds);
-    int val = read & 63;
-    int shift = 0;
+    unsigned int read = read_byte(bounds);
+    unsigned int val = read & 63;
+    unsigned int shift = 0;
     while (read & 64) {
         read = read_byte(bounds);
         shift += 6;
@@ -853,7 +853,7 @@ read_varint(PyCodeAddressRange *bounds)
 static int
 read_signed_varint(PyCodeAddressRange *bounds)
 {
-    int uval = read_varint(bounds);
+    unsigned int uval = read_varint(bounds);
     if (uval & 1) {
         return -(int)(uval >> 1);
     }