[Rd] bug [methods]: double execution of `insertSource` within the same session does not work

[Demetrio Rodriguez T.]

Hello everyone,


I hope this reaches someone at all. It's my first bug report to the R-core,
and, apparently, bugzilla is locked from new reports for now.

I was using `methods::insertSource` to debug and successfully fix another
package, until it suddenly stopped working. I figured out, that it is
because I am using it on the same function multiple times within one
session. It also produces warnings even during the first call, but somehow
still works. Below I provide a reproducible example:

SETUP:
```bash
demetrio at laptop:[folder_structure]/Bugreports/methods_insertSource$ ls -a
.  ..  gmapsdistance_fix.R  methods_insertSource.R
```

file `gmapsdistance_fix.R`
```R
gmapsdistance = function(param) {
    print('I am a bug report, my params are:')
    print(param)
}
```


file `methods_insertSource.R`
```R
library(gmapsdistance)  # works with any package

methods::insertSource('gmapsdistance_fix.R',
    package = 'gmapsdistance',
    functions = 'gmapsdistance',
    force = T
)
buggy = gmapsdistance('Works?')
```


TO REPRODUCE:
in that directory `R --vanilla` then
```R
> source('methods_insertSource.R')
Modified functions inserted through trace(): gmapsdistance
[1] "I am a bug report, my params are:"
[1] "Works?"
Warning message:
In methods::insertSource("gmapsdistance_fix.R", package = "gmapsdistance",
:
  cannot insert these (not found in source): "gmapsdistance"
# Works, but gives the warning that it does not

# repeat:
> source('methods_insertSource.R')
Error in assign(this, thisObj, envir = envwhere) :
  cannot change value of locked binding for 'gmapsdistance'
In addition: Warning message:
In methods::insertSource("gmapsdistance_fix.R", package = "gmapsdistance",
:
  cannot insert these (not found in source): "gmapsdistance"

# does not work, and gets even more confusing: so is it that the object is
not find, or is it about a locked object?
```

I think it's a bug.

--------------------- BUG REPORT END ------------------------


I looked into it a bit myself, in case you are interested:

```R
# lines 20-22
    if (is(source, "environment"))
        env <- source
    else env <- evalSource(source, package, FALSE)
    # We're in the second case I guess

# Browse[2]> env
# Object of class "sourceEnvironment"
# Source environment created  2017-12-01 05:19:51
# For package "gmapsdistance"
# From source file "gmapsdistancefix.R"


# later, before line 52:
x = env
Browse[2]> package
[1] "gmapsdistance"

# evaluate 52
packageSlot(env) <- package

# objects x and env are still identical
# Browse[2]> class(env)
# [1] "sourceEnvironment"
# attr(,"package")
# [1] "methods"
# Browse[2]> class(x)
# [1] "sourceEnvironment"
# attr(,"package")
# [1] "methods"

# Browse[2]> env
# Object of class "sourceEnvironment"
# Source environment created  2017-12-01 05:19:51
# For package "gmapsdistance"
# From source file "gmapsdistancefix.R"
# Browse[2]> x
# Object of class "sourceEnvironment"
# Source environment created  2017-12-01 05:19:51
# For package "gmapsdistance"
# From source file "gmapsdistancefix.R"

# so:
Browse[2]>  names(env)
NULL

# which is why 53-60 do not work:
allObjects <- names(env)
if (!missing(functions)) {
    notThere <- is.na(match(functions, allObjects))
    if (any(notThere)) {
        warning(gettextf("cannot insert these (not found in source): %s",
            paste("\"", functions[notThere], "\"", sep = "",
                collapse = ", ")), domain = NA)
    }
}
```

Looking forward to your feedback!

Cheers,
Demetrio

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