Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(revision 68341)
+++ Misc/NEWS	(working copy)
@@ -82,6 +82,9 @@
 Library
 -------
 
+- Issue #4842: Always append a trailing 'L' when pickling longs using
+  pickle protocol 0.  When reading, the 'L' is optional.
+
 - Issue #4272: Add an optional argument to the GzipFile constructor to override
   the timestamp in the gzip stream. The default value remains the current time.
   The information can be used by e.g. gunzip when decompressing. Patch by
Index: Lib/pickletools.py
===================================================================
--- Lib/pickletools.py	(revision 68341)
+++ Lib/pickletools.py	(working copy)
@@ -527,6 +527,8 @@
     """
 
     s = read_stringnl(f, decode=False, stripquotes=False)
+    if s[-1:] == b'L':
+        s = s[:-1]
     return int(s)
 
 
@@ -2052,39 +2054,39 @@
     1: l        LIST       (MARK at 0)
     2: p    PUT        0
     5: L    LONG       1
-    8: a    APPEND
-    9: L    LONG       2
-   12: a    APPEND
-   13: (    MARK
-   14: L        LONG       3
-   17: L        LONG       4
-   20: t        TUPLE      (MARK at 13)
-   21: p    PUT        1
-   24: a    APPEND
-   25: (    MARK
-   26: d        DICT       (MARK at 25)
-   27: p    PUT        2
-   30: c    GLOBAL     'builtins bytes'
-   46: p    PUT        3
-   49: (    MARK
-   50: (        MARK
-   51: l            LIST       (MARK at 50)
-   52: p        PUT        4
-   55: L        LONG       97
-   59: a        APPEND
-   60: L        LONG       98
+    9: a    APPEND
+   10: L    LONG       2
+   14: a    APPEND
+   15: (    MARK
+   16: L        LONG       3
+   20: L        LONG       4
+   24: t        TUPLE      (MARK at 15)
+   25: p    PUT        1
+   28: a    APPEND
+   29: (    MARK
+   30: d        DICT       (MARK at 29)
+   31: p    PUT        2
+   34: c    GLOBAL     'builtins bytes'
+   50: p    PUT        3
+   53: (    MARK
+   54: (        MARK
+   55: l            LIST       (MARK at 54)
+   56: p        PUT        4
+   59: L        LONG       97
    64: a        APPEND
-   65: L        LONG       99
-   69: a        APPEND
-   70: t        TUPLE      (MARK at 49)
-   71: p    PUT        5
-   74: R    REDUCE
-   75: p    PUT        6
-   78: V    UNICODE    'def'
-   83: p    PUT        7
-   86: s    SETITEM
-   87: a    APPEND
-   88: .    STOP
+   65: L        LONG       98
+   70: a        APPEND
+   71: L        LONG       99
+   76: a        APPEND
+   77: t        TUPLE      (MARK at 53)
+   78: p    PUT        5
+   81: R    REDUCE
+   82: p    PUT        6
+   85: V    UNICODE    'def'
+   90: p    PUT        7
+   93: s    SETITEM
+   94: a    APPEND
+   95: .    STOP
 highest protocol among opcodes = 0
 
 Try again with a "binary" pickle.
@@ -2157,12 +2159,12 @@
    92: V    UNICODE    'value'
    99: p    PUT        7
   102: L    LONG       42
-  106: s    SETITEM
-  107: b    BUILD
-  108: a    APPEND
-  109: g    GET        5
-  112: a    APPEND
-  113: .    STOP
+  107: s    SETITEM
+  108: b    BUILD
+  109: a    APPEND
+  110: g    GET        5
+  113: a    APPEND
+  114: .    STOP
 highest protocol among opcodes = 0
 
 >>> dis(pickle.dumps(x, 1))
Index: Lib/test/pickletester.py
===================================================================
--- Lib/test/pickletester.py	(revision 68341)
+++ Lib/test/pickletester.py	(working copy)
@@ -90,21 +90,21 @@
 # the object returned by create_data().
 
 DATA0 = (
-    b'(lp0\nL0\naL1\naF2.0\nac'
+    b'(lp0\nL0L\naL1L\naF2.0\nac'
     b'builtins\ncomplex\n'
     b'p1\n(F3.0\nF0.0\ntp2\nRp'
-    b'3\naL1\naL-1\naL255\naL-'
-    b'255\naL-256\naL65535\na'
-    b'L-65535\naL-65536\naL2'
-    b'147483647\naL-2147483'
-    b'647\naL-2147483648\na('
+    b'3\naL1L\naL-1L\naL255L\naL-'
+    b'255L\naL-256L\naL65535L\na'
+    b'L-65535L\naL-65536L\naL2'
+    b'147483647L\naL-2147483'
+    b'647L\naL-2147483648L\na('
     b'Vabc\np4\ng4\nccopyreg'
     b'\n_reconstructor\np5\n('
     b'c__main__\nC\np6\ncbu'
     b'iltins\nobject\np7\nNt'
     b'p8\nRp9\n(dp10\nVfoo\np1'
-    b'1\nL1\nsVbar\np12\nL2\nsb'
-    b'g9\ntp13\nag13\naL5\na.'
+    b'1\nL1L\nsVbar\np12\nL2L\nsb'
+    b'g9\ntp13\nag13\naL5L\na.'
 )
 
 # Disassembly of DATA0
@@ -113,80 +113,80 @@
     1: l        LIST       (MARK at 0)
     2: p    PUT        0
     5: L    LONG       0
-    8: a    APPEND
-    9: L    LONG       1
-   12: a    APPEND
-   13: F    FLOAT      2.0
-   18: a    APPEND
-   19: c    GLOBAL     'builtins complex'
-   37: p    PUT        1
-   40: (    MARK
-   41: F        FLOAT      3.0
-   46: F        FLOAT      0.0
-   51: t        TUPLE      (MARK at 40)
-   52: p    PUT        2
-   55: R    REDUCE
-   56: p    PUT        3
-   59: a    APPEND
-   60: L    LONG       1
-   63: a    APPEND
-   64: L    LONG       -1
-   68: a    APPEND
-   69: L    LONG       255
-   74: a    APPEND
-   75: L    LONG       -255
-   81: a    APPEND
-   82: L    LONG       -256
-   88: a    APPEND
-   89: L    LONG       65535
-   96: a    APPEND
-   97: L    LONG       -65535
-  105: a    APPEND
-  106: L    LONG       -65536
+    9: a    APPEND
+   10: L    LONG       1
+   14: a    APPEND
+   15: F    FLOAT      2.0
+   20: a    APPEND
+   21: c    GLOBAL     'builtins complex'
+   39: p    PUT        1
+   42: (    MARK
+   43: F        FLOAT      3.0
+   48: F        FLOAT      0.0
+   53: t        TUPLE      (MARK at 42)
+   54: p    PUT        2
+   57: R    REDUCE
+   58: p    PUT        3
+   61: a    APPEND
+   62: L    LONG       1
+   66: a    APPEND
+   67: L    LONG       -1
+   72: a    APPEND
+   73: L    LONG       255
+   79: a    APPEND
+   80: L    LONG       -255
+   87: a    APPEND
+   88: L    LONG       -256
+   95: a    APPEND
+   96: L    LONG       65535
+  104: a    APPEND
+  105: L    LONG       -65535
   114: a    APPEND
-  115: L    LONG       2147483647
-  127: a    APPEND
-  128: L    LONG       -2147483647
-  141: a    APPEND
-  142: L    LONG       -2147483648
-  155: a    APPEND
-  156: (    MARK
-  157: V        UNICODE    'abc'
-  162: p        PUT        4
-  165: g        GET        4
-  168: c        GLOBAL     'copyreg _reconstructor'
-  192: p        PUT        5
-  195: (        MARK
-  196: c            GLOBAL     '__main__ C'
-  208: p            PUT        6
-  211: c            GLOBAL     'builtins object'
-  228: p            PUT        7
-  231: N            NONE
-  232: t            TUPLE      (MARK at 195)
-  233: p        PUT        8
-  236: R        REDUCE
-  237: p        PUT        9
-  240: (        MARK
-  241: d            DICT       (MARK at 240)
-  242: p        PUT        10
-  246: V        UNICODE    'foo'
-  251: p        PUT        11
-  255: L        LONG       1
-  258: s        SETITEM
-  259: V        UNICODE    'bar'
-  264: p        PUT        12
-  268: L        LONG       2
-  271: s        SETITEM
-  272: b        BUILD
-  273: g        GET        9
-  276: t        TUPLE      (MARK at 156)
-  277: p    PUT        13
-  281: a    APPEND
-  282: g    GET        13
-  286: a    APPEND
-  287: L    LONG       5
-  290: a    APPEND
-  291: .    STOP
+  115: L    LONG       -65536
+  124: a    APPEND
+  125: L    LONG       2147483647
+  138: a    APPEND
+  139: L    LONG       -2147483647
+  153: a    APPEND
+  154: L    LONG       -2147483648
+  168: a    APPEND
+  169: (    MARK
+  170: V        UNICODE    'abc'
+  175: p        PUT        4
+  178: g        GET        4
+  181: c        GLOBAL     'copyreg _reconstructor'
+  205: p        PUT        5
+  208: (        MARK
+  209: c            GLOBAL     '__main__ C'
+  221: p            PUT        6
+  224: c            GLOBAL     'builtins object'
+  241: p            PUT        7
+  244: N            NONE
+  245: t            TUPLE      (MARK at 208)
+  246: p        PUT        8
+  249: R        REDUCE
+  250: p        PUT        9
+  253: (        MARK
+  254: d            DICT       (MARK at 253)
+  255: p        PUT        10
+  259: V        UNICODE    'foo'
+  264: p        PUT        11
+  268: L        LONG       1
+  272: s        SETITEM
+  273: V        UNICODE    'bar'
+  278: p        PUT        12
+  282: L        LONG       2
+  286: s        SETITEM
+  287: b        BUILD
+  288: g        GET        9
+  291: t        TUPLE      (MARK at 169)
+  292: p    PUT        13
+  296: a    APPEND
+  297: g    GET        13
+  301: a    APPEND
+  302: L    LONG       5
+  306: a    APPEND
+  307: .    STOP
 highest protocol among opcodes = 0
 """
 
Index: Lib/pickle.py
===================================================================
--- Lib/pickle.py	(revision 68341)
+++ Lib/pickle.py	(working copy)
@@ -470,7 +470,7 @@
             else:
                 self.write(LONG4 + pack("<i", n) + encoded)
             return
-        self.write(LONG + repr(obj).encode("ascii") + b'\n')
+        self.write(LONG + repr(obj).encode("ascii") + b'L\n')
     dispatch[int] = save_long
 
     def save_float(self, obj, pack=struct.pack):
@@ -890,6 +890,8 @@
 
     def load_long(self):
         val = self.readline()[:-1].decode("ascii")
+        if val and val[-1] == 'L':
+            val = val[:-1]
         self.append(int(val, 0))
     dispatch[LONG[0]] = load_long
 
Index: Modules/_pickle.c
===================================================================
--- Modules/_pickle.c	(revision 68341)
+++ Modules/_pickle.c	(working copy)
@@ -846,8 +846,8 @@
         /* Text-mode pickle, or long too big to fit in the 4-byte
          * signed BININT format:  store as a string.
          */
-        pdata[0] = LONG;        /* use LONG for consistence with pickle.py */
-        PyOS_snprintf(pdata + 1, sizeof(pdata) - 1, "%ld\n", x);
+        pdata[0] = LONG;        /* use LONG for consistency with pickle.py */
+        PyOS_snprintf(pdata + 1, sizeof(pdata) - 1, "%ldL\n", x);
         if (pickler_write(self, pdata, strlen(pdata)) < 0)
             return -1;
     }
@@ -977,8 +977,9 @@
     else {
         char *string;
 
-        /* proto < 2:  write the repr and newline.  This is quadratic-time
-           (in the number of digits), in both directions. */
+        /* proto < 2: write the repr and newline.  This is quadratic-time (in
+           the number of digits), in both directions.  We add a trailing 'L'
+           to the repr, for compatibility with Python 2.x. */
 
         repr = PyObject_Repr(obj);
         if (repr == NULL)
@@ -990,7 +991,7 @@
 
         if (pickler_write(self, &long_op, 1) < 0 ||
             pickler_write(self, string, size) < 0 ||
-            pickler_write(self, "\n", 1) < 0)
+            pickler_write(self, "L\n", 2) < 0)
             goto error;
     }
 
@@ -2880,7 +2881,7 @@
 load_long(UnpicklerObject *self)
 {
     PyObject *value;
-    char *s;
+    char *s, *ss;
     Py_ssize_t len;
 
     if ((len = unpickler_readline(self, &s)) < 0)
@@ -2888,8 +2889,27 @@
     if (len < 2)
         return bad_readline();
 
-    /* XXX: Should the base argument explicitly set to 10? */
-    if ((value = PyLong_FromString(s, NULL, 0)) == NULL)
+    /* s[len-2] will usually be 'L' (and s[len-1] is '\n'); we need to remove
+       the 'L' before calling PyLong_FromString.  In order to maintain
+       compatibility with Python 3.0.0, we don't actually *require*
+       the 'L' to be present. */
+    if (s[len-2] == 'L') {
+        ss = (char *)PyMem_Malloc(len-1);
+        if (ss == NULL) {
+            PyErr_NoMemory();
+            return -1;
+        }
+        strncpy(ss, s, len-2);
+        ss[len-2] = '\0';
+
+        /* XXX: Should the base argument explicitly set to 10? */
+        value = PyLong_FromString(ss, NULL, 0);
+        PyMem_Free(ss);
+    }
+    else {
+        value = PyLong_FromString(s, NULL, 0);
+    }
+    if (value == NULL)
         return -1;
 
     PDATA_PUSH(self->stack, value, -1);