start of error handling

.travis.yml

@@ -3,7 +3,10 @@ language: rust
 cache: cargo
 rust:
   - nightly
-  - stable
+# - beta
+# ^ to be changed once question_mark is in beta
+# - stable
+# ^ to be changed once question_mark becomes stable
 before_script:
   - (cargo install mdbook || true)
 script:

src/SUMMARY.md

@@ -36,7 +36,10 @@
     - [Strings]()
     - [`HashMap<K, V>`]()
 
-- [Error Handling]()
+- [Error Handling](ch09-00-error-handling.md)
+    - [Unrecoverable errors with panic!](ch09-01-unrecoverable-errors-with-panic.md)
+    - [Recoverable errors with `Result<T, E>`](ch09-02-recoverable-errors-with-result.md)
+    - [To `panic!` or Not To `panic!`](ch09-03-to-panic-or-not-to-panic.md)
 
 - [Lifetimes]()
 

src/ch09-00-error-handling.md

@@ -0,0 +1,26 @@
+# Error Handling
+
+Rust's focus on reliability extends to the area of error handling. Errors are a
+fact of life in software, so Rust has a number of features that you can use to
+handle situations in which something bad happens. In many cases, Rust requires
+you to acknowledge the possibility of an error occurring and take some action
+in that situation. This makes your program more robust by eliminating the
+possibility of unexpected errors only being discovered after you've deployed
+your code to production.
+
+Rust groups errors into two major kinds: errors that are *recoverable*, and
+errors that are *unrecoverable*. Recoverable errors are problems like a file not
+being found, where it's usually reasonable to report that problem to the user
+and retry the operation. Unrecoverable errors are problems like trying to
+access a location beyond the end of an array, and these are always symptoms of
+bugs.
+
+Most languages do not distinguish between the two kinds of errors, so they
+handle both kinds in the same way using mechanisms like exceptions. Rust
+doesn't have exceptions. Instead, it has the value `Result<T, E>` to return in
+the case of recoverable errors and the `panic!` macro that stops execution when
+it encounters unrecoverable errors. This chapter will cover the more
+straightforward case of calling `panic!` first. Then, we'll talk about
+returning `Result<T, E>` values and calling functions that return `Result<T,
+E>`. Finally, we'll discuss considerations to take into account when deciding
+whether to try to recover from an error or to stop execution.

src/ch09-01-unrecoverable-errors-with-panic.md

@@ -0,0 +1,145 @@
+## Unrecoverable errors with panic!
+
+Sometimes, bad things happen, and there's nothing that you can do about it. For
+these cases, Rust has a macro, `panic!`. When this macro executes, your program
+will print a failure message, unwind and clean up the stack, and then quit. The
+most common reason for this is when a bug of some kind has been detected, and
+it's not clear how to handle the error.
+
+<!-- PROD: START BOX -->
+
+> #### Unwinding
+> By default, when a `panic!` happens in Rust, the program starts
+> *unwinding*, which means Rust walks back up the stack and cleans up the data
+> from each function it encounters. Doing that walking and cleanup is a lot of
+> work. The alternative is to immediately `abort`, which ends the program
+> without cleaning up. Memory that the program was using will need to be cleaned
+> up by the operating system. If you're in a situation where you need to make
+> the resulting binary as small as possible, you can switch from unwinding on
+> panic to aborting on panic by adding `panic = 'abort'` to the appropriate
+> `[profile]` sections in your `Cargo.toml` file.
+
+<!-- PROD: END BOX -->
+
+Let's try out calling `panic!()` with a simple program:
+
+```rust,should_panic
+fn main() {
+    panic!("crash and burn");
+}
+```
+
+If you run it, you'll see something like this:
+
+```bash
+$ cargo run
+   Compiling panic v0.1.0 (file:///projects/panic)
+    Finished debug [unoptimized + debuginfo] target(s) in 0.25 secs
+     Running `target/debug/panic`
+thread 'main' panicked at 'crash and burn', src/main.rs:2
+note: Run with `RUST_BACKTRACE=1` for a backtrace.
+error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
+```
+
+There are three lines of error message here. The first line shows our panic
+message and the place in our source code where the panic occurred:
+`src/main.rs`, line two.
+
+But that only shows us the exact line that called `panic!`. That's not always
+useful. Let's look at another example to see what it's like when a `panic!`
+call comes from code we call instead of from our code directly:
+
+```rust,should_panic
+fn main() {
+    let v = vec![1, 2, 3];
+
+    v[100];
+}
+```
+
+We're attempting to access the hundredth element of our vector, but it only has
+three elements. In this situation, Rust will panic. Using `[]` is supposed to
+return an element. If you pass `[]` an invalid index, though, there's no
+element that Rust could return here that would be correct.
+
+Other languages like C will attempt to give you exactly what you asked for in
+this situation, even though it isn't what you want: you'll get whatever is at
+the location in memory that would correspond to that element in the vector,
+even though the memory doesn't belong to the vector. This is called a *buffer
+overread*, and can lead to security vulnerabilities if an attacker can
+manipulate the index in such a way as to read data they shouldn't be allowed to
+that is stored after the array.
+
+In order to protect your program from this sort of vulnerability, if you try to
+read an element at an index that doesn't exist, Rust will stop execution and
+refuse to continue with an invalid value. Let's try it and see:
+
+```bash
+$ cargo run
+   Compiling panic v0.1.0 (file:///projects/panic)
+    Finished debug [unoptimized + debuginfo] target(s) in 0.27 secs
+     Running `target/debug/panic`
+thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
+100', ../src/libcollections/vec.rs:1265
+note: Run with `RUST_BACKTRACE=1` for a backtrace.
+error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
+```
+
+This points at a file we didn't write, `../src/libcollections/vec.rs`. That's
+the implementation of `Vec<T>` in the standard library. While it's easy to see
+in this short program where the error was, it would be nicer if we could have
+Rust tell us what line in our program caused the error.
+
+That's what the next line, the `note` is about. If we set the `RUST_BACKTRACE`
+environment variable, we'll get a backtrace of exactly how the error happend.
+Let's try that. Listing 9-1 shows the output:
+
+```bash
+$ RUST_BACKTRACE=1 cargo run
+    Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
+     Running `target/debug/panic`
+thread 'main' panicked at 'index out of bounds: the len is 3 but the index is
+100', ../src/libcollections/vec.rs:1265
+stack backtrace:
+   1:     0x560956150ae9 -
+std::sys::backtrace::tracing::imp::write::h482d45d91246faa2
+   2:     0x56095615345c -
+std::panicking::default_hook::_{{closure}}::h89158f66286b674e
+   3:     0x56095615291e - std::panicking::default_hook::h9e30d428ee3b0c43
+   4:     0x560956152f88 -
+std::panicking::rust_panic_with_hook::h2224f33fb7bf2f4c
+   5:     0x560956152e22 - std::panicking::begin_panic::hcb11a4dc6d779ae5
+   6:     0x560956152d50 - std::panicking::begin_panic_fmt::h310416c62f3935b3
+   7:     0x560956152cd1 - rust_begin_unwind
+   8:     0x560956188a2f - core::panicking::panic_fmt::hc5789f4e80194729
+   9:     0x5609561889d3 -
+core::panicking::panic_bounds_check::hb2d969c3cc11ed08
+  10:     0x56095614c075 - _<collections..vec..Vec<T> as
+core..ops..Index<usize>>::index::hb9f10d3dadbe8101
+                        at ../src/libcollections/vec.rs:1265
+  11:     0x56095614c134 - panic::main::h2d7d3751fb8705e2
+                        at /projects/panic/src/main.rs:4
+  12:     0x56095615af46 - __rust_maybe_catch_panic
+  13:     0x560956152082 - std::rt::lang_start::h352a66f5026f54bd
+  14:     0x56095614c1b3 - main
+  15:     0x7f75b88ed72f - __libc_start_main
+  16:     0x56095614b3c8 - _start
+  17:                0x0 - <unknown>
+error: Process didn't exit successfully: `target/debug/panic` (exit code: 101)
+```
+
+<caption>
+Listing 9-1: The backtrace generated by a call to `panic!` displayed when
+the environment variable `RUST_BACKTRACE` is set
+</caption>
+
+That's a lot of output! Line 11 of the backtrace points to the line in our
+project causing the problem: `src/main.rs` line four. The key to reading the
+backtrace is to start from the top and read until we see files that we wrote:
+that's where the problem originated. If we didn't want our program to panic
+here, this line is where we would start investigating in order to figure out
+how we got to this location with values that caused the panic.
+
+Now that we've covered how to `panic!` to stop our code's execution and how to
+debug a `panic!`, let's look at how to instead return and use recoverable
+errors with `Result`.