fix #1904

/dev/null permissions were modified when using sudo rights.
This fixes this bug during decompression.

More importantly, this patch  adds a test, triggered in TravisCI,
ensuring unaltered /dev/null permissions.

tests/playTests.sh

@@ -220,13 +220,12 @@ $ZSTD tmp -c --compress-literals    -19      | $ZSTD -t
 $ZSTD -b --fast=1 -i0e1 tmp --compress-literals
 $ZSTD -b --fast=1 -i0e1 tmp --no-compress-literals
 
-println "test: --exclude-compressed flag"
+println "\n===>  --exclude-compressed flag"
 rm -rf precompressedFilterTestDir
 mkdir -p precompressedFilterTestDir
 ./datagen $size > precompressedFilterTestDir/input.5
 ./datagen $size > precompressedFilterTestDir/input.6
 $ZSTD --exclude-compressed --long --rm -r precompressedFilterTestDir
-sleep 5
 ./datagen $size > precompressedFilterTestDir/input.7
 ./datagen $size > precompressedFilterTestDir/input.8
 $ZSTD --exclude-compressed --long --rm -r precompressedFilterTestDir
@@ -251,7 +250,7 @@ test -f precompressedFilterTestDir/input.5.zst.zst
 test -f precompressedFilterTestDir/input.6.zst.zst
 println "Test completed"
 
-println "test : file removal"
+println "\n===>  file removal"
 $ZSTD -f --rm tmp
 test ! -f tmp  # tmp should no longer be present
 $ZSTD -f -d --rm tmp.zst
@@ -278,13 +277,16 @@ $ZSTD -f tmp && die "attempt to compress a non existing file"
 test -f tmp.zst  # destination file should still be present
 rm -rf tmp*  # may also erase tmp* directory from previous failed run
 
-println "\n===> decompression only tests "
-dd bs=1 count=1048576 if=/dev/zero of=tmp
+println "\n===>  decompression only tests "
+# the following test verifies that the decoder is compatible with RLE as first block
+# older versions of zstd cli are not able to decode such corner case.
+# As a consequence, the zstd cli do not generate them, to maintain compatibility with older versions.
+dd bs=1048576 count=1 if=/dev/zero of=tmp
 $ZSTD -d -o tmp1 "$TESTDIR/golden-decompression/rle-first-block.zst"
 $DIFF -s tmp1 tmp
 rm tmp*
 
-println "test : compress multiple files"
+println "\m===>  compress multiple files"
 println hello > tmp1
 println world > tmp2
 $ZSTD tmp1 tmp2 -o "$INTOVOID" -f
@@ -306,7 +308,17 @@ if [ "$?" -eq 139 ]; then
 fi
 rm tmp*
 
-println "test : compress multiple files into an output directory, --output-dir-flat"
+if [ -n "$DEVNULLRIGHTS" ]
+then
+    # these tests requires sudo rights, which is uncommon.
+    # they are only triggered if DEVNULLRIGHTS macro is defined.
+    println "\n===> checking /dev/null permissions are unaltered "
+    ./datagen > tmp
+    sudo $ZSTD tmp -o $INTOVOID   # sudo rights could modify /dev/null permissions
+    ls -las $INTOVOID | grep "rw-rw-rw-"
+fi
+
+println "\n===>  compress multiple files into an output directory, --output-dir-flat"
 println henlo > tmp1
 mkdir tmpInputTestDir
 mkdir tmpInputTestDir/we
@@ -352,7 +364,6 @@ $ZSTD -dcf tmp1
 
 
 println "\n===>  frame concatenation "
-
 println "hello " > hello.tmp
 println "world!" > world.tmp
 cat hello.tmp world.tmp > helloworld.tmp