Fix buck dev mode fbcode builds (#4747)

Summary:
Don't enable ROCKSDB_JEMALLOC unless the build mode is opt and default
allocator is jemalloc. In dev mode, this is causing compile/link errors such as -
```
stderr: buck-out/dev/gen/rocksdb/src/rocksdb_lib#compile-pic-malloc_stats.cc.o4768b59e,gcc-5-glibc-2.23-clang/db/malloc_stats.cc.o:malloc_stats.cc:function rocksdb::DumpMallocStats(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*): error: undefined reference to 'malloc_stats_print'
clang-7.0: error: linker command failed with exit code 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4747

Differential Revision: D13324840

Pulled By: anand1976

fbshipit-source-id: 45ffbd4f63fe4d9e8a0473d8f066155e4ef64a14

TARGETS

@@ -73,7 +73,7 @@ sanitizer = read_config("fbcode", "sanitizer")
 
 # Let RocksDB aware of jemalloc existence.
 # Do not enable it if sanitizer presents.
-if default_allocator.startswith("jemalloc") and sanitizer == "":
+if is_opt_mode and default_allocator.startswith("jemalloc") and sanitizer == "":
     rocksdb_compiler_flags.append("-DROCKSDB_JEMALLOC")
     rocksdb_external_deps.append(("jemalloc", None, "headers"))
 
@@ -990,6 +990,11 @@ ROCKS_TESTS = [
         "tools/sst_dump_test.cc",
         "serial",
     ],
+    [
+        "sst_file_reader_test",
+        "table/sst_file_reader_test.cc",
+        "serial",
+    ],
     [
         "statistics_test",
         "monitoring/statistics_test.cc",
@@ -1100,11 +1105,6 @@ ROCKS_TESTS = [
         "utilities/transactions/write_unprepared_transaction_test.cc",
         "parallel",
     ],
-    [
-        "sst_file_reader_test",
-        "table/sst_file_reader_test.cc",
-        "serial",
-    ],
 ]
 
 # Generate a test rule for each entry in ROCKS_TESTS

buckifier/targets_cfg.py

@@ -77,7 +77,7 @@ sanitizer = read_config("fbcode", "sanitizer")
 
 # Let RocksDB aware of jemalloc existence.
 # Do not enable it if sanitizer presents.
-if default_allocator.startswith("jemalloc") and sanitizer == "":
+if is_opt_mode and default_allocator.startswith("jemalloc") and sanitizer == "":
     rocksdb_compiler_flags.append("-DROCKSDB_JEMALLOC")
     rocksdb_external_deps.append(("jemalloc", None, "headers"))
 """