Add reoccuring errors doc
There are a number of common coding, design, and implementation mistakes
that bmcweb users tend to make. This attempts to document them so we
have a single source to point users to when making gerrit comments. The
hope is that this alleviates some of the early mistakes that new users
tend to make.
Tested:
Documentation only. No functional changes.
Signed-off-by: Ed Tanous <ed@tanous.net>
Change-Id: Icc0081c94403c937d9a1ce44b7d6e81a5716a32e
diff --git a/COMMON_ERRORS.md b/COMMON_ERRORS.md
new file mode 100644
index 0000000..c68c86c
--- /dev/null
+++ b/COMMON_ERRORS.md
@@ -0,0 +1,181 @@
+# Commonly recurring errors in bmcweb
+
+What follows is a list of common errors that new users to bmcweb tend to make
+when operating within its bounds for the first time. If this is your first time
+developing in bmcweb, the maintainers highly recommend reading and understanding
+_all_ of common traps before continuing with any development. Every single one
+of the examples below compile without warnings, but are incorrect in
+not-always-obvious ways, or impose a pattern that tends to cause hard to find
+bugs, or bugs that appear later. Every one has been submitted to code review
+multiple times.
+
+### Directly dereferencing a pointer without checking for validity first
+```C++
+int myBadMethod(const nlohmann::json& j){
+ const int* myPtr = j.get_if<int>();
+ return *myPtr;
+}
+```
+This pointer is not guaranteed to be filled, and could be a null dereference.
+
+### String views aren't null terminated
+```C++
+int getIntFromString(const std::string_view s){
+ return std::atoi(s.data());
+}
+```
+This will give the right answer much of the time, but has the possibility to
+fail when string\_view is not null terminated. Use from\_chars instead, which
+takes both a pointer and a length
+
+### Not handling input errors
+```C++
+int getIntFromString(const std::string& s){
+ return std::atoi(s.c_str());
+}
+```
+In the case where the string is not representable as an int, this will trigger
+undefined behavior at system level. Code needs to check for validity of the
+string, ideally with something like from\_chars, and return the appropriate error
+code.
+
+### Walking off the end of a string
+```C++
+std::string getFilenameFromPath(const std::string& path){
+ size_t index = path.find("/");
+ if (index != std::string::npos){
+ // If the string ends with "/", this will walk off the end of the string.
+ return path.substr(pos + 1);
+ }
+ return "";
+}
+```
+
+### Using methods that throw (or not handling bad inputs)
+```C++
+int myBadMethod(nlohmann::json& j){
+ return j.get<int>();
+}
+```
+This method throws, and bad inputs will not be handled
+
+Commonly used methods that fall into this pattern:
+std::variant::get
+std::vector::at
+std::map::at
+std::set::at
+std::<generic container type>::at
+nlohmann::json::operator!=
+nlohmann::json::operator+=
+nlohmann::json::at
+nlohmann::json::get
+nlohmann::json::get\_ref
+nlohmann::json::get\_to
+std::filesystem::create\_directory
+std::filesystem::rename
+std::filesystem::file\_size
+std::stoi
+std::stol
+std::stoll
+
+#### special/strange case:
+
+nlohmann::json::parse by default throws on failure, but also accepts a optional
+argument that causes it to not throw. Please consult the other examples in the
+code for how to handle errors.
+
+
+#### Special note: Boost
+there is a whole class of boost asio functions that provide both a method that
+throws on failure, and a method that accepts and returns an error code. This is
+not a complete list, but users should verify in the boost docs when calling into
+asio methods, and prefer the one that returns an error code instead of throwing.
+
+boost::asio::ip::tcp::acceptor::bind();
+boost::asio::ip::tcp::acceptor::cancel();
+boost::asio::ip::tcp::acceptor::close();
+boost::asio::ip::tcp::acceptor::listen();
+boost::asio::ip::address::make\_address();
+
+### Blocking functions
+
+bmcweb uses a single reactor for all operations. Blocking that reactor for any
+amount of time causes all other operations to stop. The common blocking
+functions that tend to be called incorrectly are:
+
+sleep()
+boost::asio::ip::tcp::socket::read()
+boost::asio::ip::tcp::socket::read\_some()
+boost::asio::ip::tcp::socket::write()
+boost::asio::ip::tcp::socket::write\_some()
+boost::asio::ip::tcp::socket::connect()
+boost::asio::ip::tcp::socket::send()
+boost::asio::ip::tcp::socket::wait()
+boost::asio::steady\_timer::wait()
+
+Note: an exception is made for filesystem/disk IO read and write. This is
+mostly due to not having great abstractions for it that mate well with the async
+system, the fact that most filesystem accesses are into tmpfs (and therefore
+should be "fast" most of the time) and in general how little the filesystem is
+used in practice.
+
+### Lack of locking between subsequent calls
+While global data structures are discouraged, they are sometimes required to
+store temporary state for operations that require it. Given the single
+threaded nature of bmcweb, they are not required to be explicitly threadsafe,
+but they must be always left in a valid state, and checked for other uses
+before occupying.
+
+```C++
+std::optional<std::string> currentOperation;
+void firstCallbackInFlow(){
+ currentOperation = "Foo";
+}
+void secondCallbackInFlow(){
+ currentOperation.reset();
+}
+```
+
+In the above case, the first callback needs a check to ensure that
+currentOperation is not already being used.
+
+### Wildcard reference captures in lambdas
+```
+std::string x; auto mylambda = [&](){
+ x = "foo";
+}
+do_async_read(mylambda)
+```
+
+Numerous times, lifetime issues of const references have been injected into
+async bmcweb code. While capturing by reference can be useful, given how
+difficult these types of bugs are to triage, bmcweb explicitly requires that all
+code captures variables by name explicitly, and calls out each variable being
+captured by value or by reference. The above prototypes would change to
+[&x]()... Which makes clear that x is captured, and its lifetime needs tracked.
+
+
+### URLs should end in "/"
+```C++
+BMCWEB("/foo/bar");
+```
+Unless you explicitly have a reason not to (as there is one known exception
+where the behavior must differ) all URL handlers should end in "/". The bmcweb
+route handler will detect routes ending in slash and generate routes for both
+the route ending in slash and the one without. This allows both URLs to be
+used by users. While many specifications do not require this, it resolves a
+whole class of bug that we've seen in the past.
+
+
+### URLs constructed in aggregate
+```C++
+std::string routeStart = "/redfish/v1";
+
+BMCWEB_ROUTE(routestart + "/SessionService/")
+```
+Very commonly, bmcweb maintainers and contributors alike have to do audits of
+all routes that are available, to verify things like security and documentation
+accuracy. While these processes are largely manual, they can mostly be
+conducted by a simple grep statement to search for urls in question. Doing the
+above makes the route handlers no longer greppable, and complicates bmcweb
+patchsets as a whole.
diff --git a/DEVELOPING.md b/DEVELOPING.md
index 8c7ceab..6e5e1ed 100644
--- a/DEVELOPING.md
+++ b/DEVELOPING.md
@@ -126,7 +126,13 @@
meetings. For more information on Redfish and supported schemas visit
[Redfish.md](https://github.com/openbmc/bmcweb/blob/master/Redfish.md).
-13. ### Developing and Testing
+13. ### Common errors
+ A number of examples of common errors are captured in the common errors doc.
+ It is recommended that developers read and understand all of them before
+ starting any openbmc development.
+ [Common Errors](https://github.com/openbmc/bmcweb/blob/master/COMMON_ERRORS.md).
+
+14. ### Developing and Testing
There are a variety of ways to develop and test bmcweb software changes.
Here are the steps for using the SDK and QEMU.