followSymlinks: Improve error message

.gitignore

@@ -43,6 +43,9 @@ perl/Makefile.config
 /src/libexpr/nix.tbl
 /src/libexpr/tests/libnixexpr-tests
 
+# /src/libfetchers
+/src/libfetchers/tests/libnixfetchers-tests
+
 # /src/libstore/
 *.gen.*
 /src/libstore/tests/libnixstore-tests

Makefile

@@ -29,6 +29,7 @@ UNIT_TEST_ENV = _NIX_TEST_UNIT_DATA=unit-test-data
 makefiles += \
   src/libutil/tests/local.mk \
   src/libstore/tests/local.mk \
+  src/libfetchers/tests/local.mk \
   src/libexpr/tests/local.mk
 endif
 

src/libfetchers/input-accessor.cc

@@ -70,7 +70,7 @@ SourcePath SourcePath::followSymlinks() const {
     while (true) {
         // Basic cycle/depth limit to avoid infinite loops.
         if (++followCount >= maxFollow)
-            throw Error("too many symbolic links encountered while traversing the path '%s'", path);
+            throw Error("too many levels of symbolic links while traversing the path '%s'; assuming it leads to a cycle after following %d indirections", this->to_string(), maxFollow);
         if (path.lstat().type != InputAccessor::tSymlink) break;
         path = {path.accessor, CanonPath(path.readLink(), path.path.parent().value_or(CanonPath::root))};
     }

src/libfetchers/memory-input-accessor.cc

@@ -12,6 +12,14 @@ struct MemoryInputAccessorImpl : MemoryInputAccessor, MemorySourceAccessor
             MemorySourceAccessor::addFile(path, std::move(contents))
         };
     }
+
+    SourcePath addSymlink(CanonPath path, std::string && contents) override
+    {
+        return {
+            ref(shared_from_this()),
+            MemorySourceAccessor::addSymlink(path, std::move(contents))
+        };
+    }
 };
 
 ref<MemoryInputAccessor> makeMemoryInputAccessor()

src/libfetchers/memory-input-accessor.hh

@@ -8,6 +8,7 @@ namespace nix {
 struct MemoryInputAccessor : InputAccessor
 {
     virtual SourcePath addFile(CanonPath path, std::string && contents) = 0;
+    virtual SourcePath addSymlink(CanonPath path, std::string && contents) = 0;
 };
 
 ref<MemoryInputAccessor> makeMemoryInputAccessor();

src/libfetchers/tests/input-accessor.cc

@@ -0,0 +1,29 @@
+#include "../input-accessor.hh"
+#include "../memory-input-accessor.hh"
+#include "gmock/gmock.h"
+#include <gtest/gtest.h>
+#include "terminal.hh"
+
+namespace nix {
+
+TEST(SourcePath, followSymlinks_cycle) {
+    auto fs = makeMemoryInputAccessor();
+    fs->addSymlink({"origin", CanonPath::root}, "a");
+    fs->addSymlink({"a", CanonPath::root}, "b");
+    fs->addSymlink({"b", CanonPath::root}, "a");
+
+    ASSERT_TRUE(fs->pathExists({"a", CanonPath::root}));
+    SourcePath origin (fs->root() + "origin");
+    try {
+        origin.followSymlinks();
+        ASSERT_TRUE(false);
+    } catch (const Error &e) {
+        auto msg = filterANSIEscapes(e.what(), true);
+        // EXPECT_THAT(msg, ("too many levels of symbolic links"));
+        EXPECT_THAT(msg, testing::HasSubstr("too many levels of symbolic links"));
+        EXPECT_THAT(msg, testing::HasSubstr("'/origin'"));
+        EXPECT_THAT(msg, testing::HasSubstr("assuming it leads to a cycle after following 1000 indirections"));
+    }
+}
+
+}

src/libfetchers/tests/local.mk

@@ -0,0 +1,39 @@
+check: libfetchers-tests-exe_RUN
+
+programs += libfetchers-tests-exe
+
+libfetchers-tests-exe_NAME = libnixfetchers-tests
+
+libfetchers-tests-exe_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libfetchers-tests-exe_INSTALL_DIR := $(checkbindir)
+else
+  libfetchers-tests-exe_INSTALL_DIR :=
+endif
+
+
+libfetchers-tests-exe_LIBS = libfetchers-tests libstore
+
+
+libfetchers-tests-exe_LDFLAGS := $(GTEST_LIBS)
+
+libraries += libfetchers-tests
+
+libfetchers-tests_NAME = libnixfetchers-tests
+
+libfetchers-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libfetchers-tests_INSTALL_DIR := $(checklibdir)
+else
+  libfetchers-tests_INSTALL_DIR :=
+endif
+
+libfetchers-tests_SOURCES := $(wildcard $(d)/*.cc)
+
+libfetchers-tests_CXXFLAGS += -I src/libfetchers -I src/libutil -I src/libstore
+
+libfetchers-tests_LIBS = libutil-tests libstore-tests libutil libstore libfetchers
+
+libfetchers-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS)

src/libutil/memory-source-accessor.cc

@@ -121,6 +121,19 @@ CanonPath MemorySourceAccessor::addFile(CanonPath path, std::string && contents)
     return path;
 }
 
+CanonPath MemorySourceAccessor::addSymlink(CanonPath path, std::string &&contents)
+{
+    auto * f = open(path, File { File::Symlink {} });
+    if (!f)
+        throw Error("file '%s' cannot be made because some parent file is not a directory", path);
+    if (auto * s = std::get_if<File::Symlink>(&f->raw))
+        s->target = std::move(contents);
+    else
+        throw Error("file '%s' is not a symbolic link", path);
+
+    return path;
+}
+
 
 using File = MemorySourceAccessor::File;
 

src/libutil/memory-source-accessor.hh

@@ -70,6 +70,7 @@ struct MemorySourceAccessor : virtual SourceAccessor
     File * open(const CanonPath & path, std::optional<File> create);
 
     CanonPath addFile(CanonPath path, std::string && contents);
+    CanonPath addSymlink(CanonPath path, std::string && contents);
 };
 
 /**