AP_Filesystem: test ROMFS entries with names too long for a directory entry

Reading SITL's 300 character ROMFS names must give names cut short and
terminated, and ending the directory's name at its separator must not
write into another open listing's entry. Both fail against the readdir()
these follow.

libraries/AP_Filesystem/tests/test_romfs_dirent.cpp

@@ -0,0 +1,66 @@
+/*
+  a ROMFS entry named longer than a directory entry holds must come back cut
+  short and terminated, and reading it must not write outside its own entry.
+  SITL embeds two such entries under autotest_fixtures/long_names, a file and
+  a directory, each with a 300 character name (see Tools/ardupilotwaf/boards.py)
+ */
+
+#include <AP_gtest.h>
+
+#include <AP_Filesystem/AP_Filesystem.h>
+#include <AP_HAL/AP_HAL.h>
+
+#include <string.h>
+
+const AP_HAL::HAL& hal = AP_HAL::get_HAL();
+
+// the fixtures these need are embedded by SITL's board configuration, and
+// ROMFS itself is enabled wherever a board embeds anything at all
+#if AP_FILESYSTEM_ROMFS_ENABLED && CONFIG_HAL_BOARD == HAL_BOARD_SITL
+
+TEST(ROMFSDirent, LongNamesAreTerminated)
+{
+    auto *dir = AP::FS().opendir("@ROMFS/autotest_fixtures/long_names");
+    ASSERT_NE(dir, nullptr);
+    unsigned count = 0;
+    const struct dirent *de;
+    while ((de = AP::FS().readdir(dir)) != nullptr) {
+        count++;
+        EXPECT_LT(strnlen(de->d_name, sizeof(de->d_name)), sizeof(de->d_name));
+    }
+    AP::FS().closedir(dir);
+    EXPECT_EQ(count, 2U);
+}
+
+/*
+  the directory's name is ended at its separator, 300 characters in, which
+  is past the end of d_name.  with another listing open alongside, that
+  write lands in the entry the other listing last returned.  the other
+  listing is of autotest_fixtures, whose first entry is long_names, a name
+  long enough to show the write cutting it short
+ */
+TEST(ROMFSDirent, ALongDirectoryNameLeavesOtherListingsAlone)
+{
+    auto *long_names = AP::FS().opendir("@ROMFS/autotest_fixtures/long_names");
+    ASSERT_NE(long_names, nullptr);
+    auto *other = AP::FS().opendir("@ROMFS/autotest_fixtures");
+    ASSERT_NE(other, nullptr);
+
+    const struct dirent *other_de = AP::FS().readdir(other);
+    ASSERT_NE(other_de, nullptr);
+    char other_name[sizeof(other_de->d_name)];
+    strncpy(other_name, other_de->d_name, sizeof(other_name));
+    other_name[sizeof(other_name)-1] = 0;
+    ASSERT_STREQ(other_name, "long_names");
+
+    while (AP::FS().readdir(long_names) != nullptr) {
+    }
+    EXPECT_STREQ(other_de->d_name, other_name);
+
+    AP::FS().closedir(other);
+    AP::FS().closedir(long_names);
+}
+
+#endif  // AP_FILESYSTEM_ROMFS_ENABLED && CONFIG_HAL_BOARD == HAL_BOARD_SITL
+
+AP_GTEST_MAIN()