Add some about ownership to API-CONVENTIONS.

In particular, we often don't say anything if using the "default" conventions,
but we don't say what those are.  We've also never documented
get0/get1/set0/set1.

Change-Id: I113593cfcdcb93e0009796649e4c19219e765caf
Reviewed-on: https://boringssl-review.googlesource.com/31504
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

API-CONVENTIONS.md

@@ -98,7 +98,10 @@ objects. `bssl::UniquePtr<T>`, like other types, is forward-declared in
 `openssl/base.h`. Code that needs access to the free functions, such as code
 which destroys a `bssl::UniquePtr`, must include the corresponding module's
 header. (This matches `std::unique_ptr`'s relationship with forward
-declarations.)
+declarations.) Note, despite the name, `bssl::UniquePtr` is also used with
+reference-counted types. It owns a single reference to the object. To take an
+additional reference, use the `bssl::UpRef` function, which will return a
+separate `bssl::UniquePtr`.
 
 
 ### Stack-allocated types
@@ -175,6 +178,62 @@ These are usually for low-level cryptographic operations. These types may be
 used freely without special cleanup conventions.
 
 
+### Ownership and lifetime
+
+When working with allocated objects, it is important to think about *ownership*
+of each object, or what code is responsible for releasing it. This matches the
+corresponding notion in higher-level languages like C++ and Rust. Ownership
+applies to both uniquely-owned types and reference-counted types. For the
+latter, ownership means the code is responsible for releasing one reference.
+
+As BoringSSL's APIs are primarily in C, ownership and lifetime obligations are
+not rigorously annotated in the type signatures or checked at compile-time.
+Instead, they are described in
+[API documentation](https://commondatastorage.googleapis.com/chromium-boringssl-docs/headers.html).
+This section describes some conventions.
+
+Unless otherwise documented, functions do not take ownership of pointer
+arguments. The pointer typically must remain valid for the duration of the
+function call. The function may internally copy information from the argument or
+take a reference, but the caller is free to release its copy or reference at any
+point after the call completes.
+
+A function may instead be documented to *take* or *transfer* ownership of a
+pointer. The caller must own the object before the function call and, after
+transfer, no longer owns it. As a corollary, the caller may no longer reference
+the object without a separate guarantee on the lifetime. The function may even
+release the object before returning. Callers that wish to independently retain a
+transfered object must therefore take a reference or make a copy before
+transferring. Callers should also take note of whether the function is
+documented to transfer pointers unconditionally or only on success. Unlike C++
+and Rust, functions in BoringSSL typically only transfer on success.
+
+Likewise, output pointers may be owning or non-owning. Unless otherwise
+documented, functions output non-owning pointers. The caller is not responsible
+for releasing the output pointer, but it must not use the pointer beyond its
+lifetime. The pointer may be released when the parent object is released or even
+sooner on state change in the parent object.
+
+If documented to output a *newly-allocated* object or a *reference* or *copy* of
+one, the caller is responsible for releasing the object when it is done.
+
+By convention, functions named `get0` return non-owning pointers. Functions name
+`new` or `get1` return owning pointers. Functions named `set0` take ownership of
+arguments. Functions named `set1` do not. They typically take a reference or
+make a copy internally. These names originally referred to the effect on a
+reference count, but the convention applies equally to non-reference-counted
+types.
+
+API documentation may also describe more complex obligations. For instance, an
+object may borrow a pointer for longer than the duration of a single function
+call, in which case the caller must ensure the lifetime extends accordingly.
+
+Memory errors are one of the most common and dangerous bugs in C and C++, so
+callers are encouraged to make use of tools such as
+[AddressSanitizer](https://clang.llvm.org/docs/AddressSanitizer.html) and
+higher-level languages.
+
+
 ## Thread safety
 
 BoringSSL is internally aware of the platform threading library and calls into