Fix corruption that rarely occurs in 32-bit mode with wlog=25

Fix an off-by-one error in the compressor that emits corrupt blocks if:

* Zstd is compiled in 32-bit mode
* The windowLog == 25 exactly
* An offset of 2^25-3, 2^25-2, 2^25-1, or 2^25 is emitted
* The bitstream had 7 bits leftover before writing the offset

This bug has been present since before v1.0, but wasn't able to easily
be triggered, since until somewhat recently zstd wasn't able to find
matches that were within 128KB of the window size.

Add a test case, and fix 2 bugs in `ZSTD_compressSequences()`:
* The `ZSTD_isRLE()` check was incorrect. It wouldn't produce
  corruption, but it could waste CPU and not emit RLE even if the block
  was RLE
* One windowSize was `1 << windowLog`, not `1u << windowLog`

Thanks to @tansy for finding the issue, and giving us a reproducer!

Fixes Issue #3350.

lib/compress/zstd_compress.c

@@ -2624,7 +2624,7 @@ ZSTD_entropyCompressSeqStore_internal(seqStore_t* seqStorePtr,
                                 void* entropyWorkspace, size_t entropyWkspSize,
                           const int bmi2)
 {
-    const int longOffsets = cctxParams->cParams.windowLog > STREAM_ACCUMULATOR_MIN;
+    const int longOffsets = cctxParams->cParams.windowLog >= STREAM_ACCUMULATOR_MIN;
     ZSTD_strategy const strategy = cctxParams->cParams.strategy;
     unsigned* count = (unsigned*)entropyWorkspace;
     FSE_CTable* CTable_LitLength = nextEntropy->fse.litlengthCTable;
@@ -5897,7 +5897,7 @@ static size_t
 ZSTD_validateSequence(U32 offCode, U32 matchLength,
                       size_t posInSrc, U32 windowLog, size_t dictSize)
 {
-    U32 const windowSize = 1 << windowLog;
+    U32 const windowSize = 1u << windowLog;
     /* posInSrc represents the amount of data the decoder would decode up to this point.
      * As long as the amount of data decoded is less than or equal to window size, offsets may be
      * larger than the total length of output decoded in order to reference the dict, even larger than
@@ -6256,7 +6256,7 @@ ZSTD_compressSequences_internal(ZSTD_CCtx* cctx,
 
         if (!cctx->isFirstBlock &&
             ZSTD_maybeRLE(&cctx->seqStore) &&
-            ZSTD_isRLE((BYTE const*)src, srcSize)) {
+            ZSTD_isRLE(ip, blockSize)) {
             /* We don't want to emit our first block as a RLE even if it qualifies because
             * doing so will cause the decoder (cli only) to throw a "should consume all input error."
             * This is only an issue for zstd <= v1.4.3

tests/zstreamtest.c

@@ -259,7 +259,7 @@ static U32 badParameters(ZSTD_CCtx* zc, ZSTD_parameters const savedParams)
     return 0;
 }
 
-static int basicUnitTests(U32 seed, double compressibility)
+static int basicUnitTests(U32 seed, double compressibility, int bigTests)
 {
     size_t const CNBufferSize = COMPRESSIBLE_NOISE_LENGTH;
     void* CNBuffer = malloc(CNBufferSize);
@@ -1565,6 +1565,62 @@ static int basicUnitTests(U32 seed, double compressibility)
     CHECK(!ZSTD_isError(ZSTD_CCtx_setParameter(zc, ZSTD_c_srcSizeHint, -1)), "Out of range doesn't error");
     DISPLAYLEVEL(3, "OK \n");
 
+    DISPLAYLEVEL(3, "test%3i : Test offset == windowSize : ", testNb++);
+    {
+        int windowLog;
+        int const kMaxWindowLog = bigTests ? 29 : 26;
+        size_t const kNbSequences = 10000;
+        size_t const kMaxSrcSize = (1u << kMaxWindowLog) + 10 * kNbSequences;
+        char* src = calloc(kMaxSrcSize, 1);
+        ZSTD_Sequence* sequences = malloc(sizeof(ZSTD_Sequence) * kNbSequences);
+        for (windowLog = ZSTD_WINDOWLOG_MIN; windowLog <= kMaxWindowLog; ++windowLog) {
+            size_t const srcSize = ((size_t)1 << windowLog) + 10 * (kNbSequences - 1);
+
+            sequences[0].offset = 32;
+            sequences[0].litLength = 32;
+            sequences[0].matchLength = (1u << windowLog) - 32;
+            sequences[0].rep = 0;
+            {
+                size_t i;
+                for (i = 1; i < kNbSequences; ++i) {
+                    sequences[i].offset = (1u << windowLog) - (FUZ_rand(&seed) % 8);
+                    sequences[i].litLength = FUZ_rand(&seed) & 7;
+                    sequences[i].matchLength = 10 - sequences[i].litLength;
+                    sequences[i].rep = 0;
+                }
+            }
+
+            CHECK_Z(ZSTD_CCtx_reset(zc, ZSTD_reset_session_and_parameters));
+            CHECK_Z(ZSTD_CCtx_setParameter(zc, ZSTD_c_checksumFlag, 1));
+            CHECK_Z(ZSTD_CCtx_setParameter(zc, ZSTD_c_minMatch, 3));
+            CHECK_Z(ZSTD_CCtx_setParameter(zc, ZSTD_c_validateSequences, 1));
+            CHECK_Z(ZSTD_CCtx_setParameter(zc, ZSTD_c_windowLog, windowLog));
+            assert(srcSize <= kMaxSrcSize);
+            cSize = ZSTD_compressSequences(zc, compressedBuffer, compressedBufferSize, sequences, kNbSequences, src, srcSize);
+            CHECK_Z(cSize);
+            CHECK_Z(ZSTD_DCtx_reset(zd, ZSTD_reset_session_and_parameters));
+            CHECK_Z(ZSTD_DCtx_setParameter(zd, ZSTD_d_windowLogMax, windowLog))
+            {
+                ZSTD_inBuffer in = {compressedBuffer, cSize, 0};
+                size_t decompressedBytes = 0;
+                for (;;) {
+                    ZSTD_outBuffer out = {decodedBuffer, decodedBufferSize, 0};
+                    size_t const ret = ZSTD_decompressStream(zd, &out, &in);
+                    CHECK_Z(ret);
+                    CHECK(decompressedBytes + out.pos > srcSize, "Output too large");
+                    CHECK(memcmp(out.dst, src + decompressedBytes, out.pos), "Corrupted");
+                    decompressedBytes += out.pos;
+                    if (ret == 0) {
+                        break;
+                    }
+                }
+                CHECK(decompressedBytes != srcSize, "Output wrong size");
+            }
+        }
+        free(src);
+    }
+    DISPLAYLEVEL(3, "OK \n");
+
     /* Overlen overwriting window data bug */
     DISPLAYLEVEL(3, "test%3i : wildcopy doesn't overwrite potential match data : ", testNb++);
     {   /* This test has a window size of 1024 bytes and consists of 3 blocks:
@@ -2677,7 +2733,7 @@ int main(int argc, const char** argv)
     if (nbTests<=0) nbTests=1;
 
     if (testNb==0) {
-        result = basicUnitTests(0, ((double)proba) / 100);  /* constant seed for predictability */
+        result = basicUnitTests(0, ((double)proba) / 100, bigTests);  /* constant seed for predictability */
     }
 
     if (!result) {