Dealing with duplicates

The linker hates it when it finds two symbol that have the same name. For example, this is an error:


#![allow(unused)]
fn main() {
#[no_mangle]
static X: u32 = 0;

#[export_name = "X"]
static Y: u32 = 0; //~ error: symbol `X` is already defined
}

This produces two symbols with the name "X". rustc catches this issue early and reports an error at compile time.

How can this occur in logging? The user may write:


#![allow(unused)]
fn main() {
extern crate defmt;
fn foo() {
    defmt::info!("foo started ..");
    // ..
    defmt::info!(".. DONE"); // <-
}

fn bar() {
    defmt::info!("bar started ..");
    // ..
    defmt::info!(".. DONE"); // <-
}
}

Because macros are expanded in isolation each info!(".. DONE") statement will produce this to intern its string:


#![allow(unused)]
fn main() {
#[export_name = ".. DONE"]
#[link_section = ".."]
static SYM: u8 = 0;
}

which results in a collision.

To avoid this issue we store each interned string as a JSON object with 3 fields: the message itself, the name of the crate that invoked the macro, and a 64-bit integer "discriminator". The discriminator is a hash of the source code location of the log statement so it should be unique per crate. Now these two macro invocations will produce something like this:


#![allow(unused)]
fn main() {
// first info! invocation
{
    #[export_name = "{ \"package\": \"my-app\", \"data\": \".. DONE\", \"discriminator\": \"1379186119\" }"]
    #[link_section = ".."]
    static SYM: u8 = 0;
}

// ..

// second info! invocation
{
    #[export_name = "{ \"package\": \"my-app\", \"data\": \".. DONE\", \"discriminator\": \"346188945\" }"]
    #[link_section = ".."]
    static SYM: u8 = 0;
}
}

These symbols do not collide because their discriminator fields are different so the program will link correctly.

Because duplicate strings are kept in the final binary this linker-based interner is not really an interner. A proper interner returns the same index when the same string is interned several times.

However, two log statements that log the same string will often have different source code locations. Assigning a different interner index to each log statement means we can distinguish between the two thus we can report their correct source code location.